ref – https://www.grammar-monster.com/glossary/subject.htm

(Issue 1) Modifiers get between the simple subject and its verb and confuse writers.

Sentences can get complicated, but writers are pretty good at making the subject and the main verb agree.

Simon, who is the oldest of the four brothers and who, just as he did before last year’s contest, has been suffering back spasms, is expected to take the first leg.
The biggest issue occurs with shorter constructions, typically in a format like a list of ideas or a range of factors.

A container of nuts and bolts were found in the cellar. (X)

(This is wrong. It should be was. The simple subject is container, which is singular.)

A range of factors have been considered. (X)

(This is wrong. It should be has. The simple subject is range, which is singular.)

(Issue 2) Terms like as well as do not form a compound subject.

Terms like as well as, along with, and together with do not compound the subject like and does.

Jack and his son are visiting tomorrow. (√)
(The word and creates a compound subject.)

Jack together with his son is visiting tomorrow. (√)
(The terms together with does not create a compound subject.)

(Issue 3) Or and nor do not conjoin.

Unlike and, the conjunctions or and nor do not conjoin.
Jack or his daughter is visiting tomorrow. (√)

Compare that with these:
Jack and his daughter are visiting tomorrow. (√)

Neither Jack nor his daughter are visiting tomorrow. (X)

(This should be is because nor does not conjoin; i.e., or does not add to the number of the subject.)

There’s a quirk though. Look at this example:
Neither Jack nor his daughters are visiting tomorrow. (√)

(This is correct because one of the nouns in the compound subject is plural.)

The example above sounds right because the noun nearest the verb (daughters) is plural.

To some, it sounds awkward when the plural noun is the first one.

Neither his daughters nor Jack are visiting tomorrow. (√)
(This is correct for the same reason; i.e., one of the nouns in the compound subject is plural.)
The words or and nor (called conjunctions) usually appear in the pairings either/or and neither/nor (called correlative conjunctions).

Proximity Rule

You should also be aware that there is a reasonably well-followed rule called the Proximity Rule, which offers different guidance.

Under the Proximity Rule, the verb is determined by the nearest noun to the verb.

Neither cakes nor chocolate is going to give you the nutrients you need.

(This is correct under the Proximity Rule because chocolate (singular) is the nearest noun to the verb, but it is wrong under the standard ruling because cakes (plural) is part of the compound subject.)

So, should you follow the standard ruling or the Proximity Rule?

For consistency, adopt the same convention as those around you. If you can’t find any examples, pick one that doesn’t grate on your ear and be consistent.

Here’s a good tip: Reword your subject to adhere to both rules.

Neither chocolate nor cakes are going to give you the nutrients you need. (√)
(Now both rules are satisfied.)

(Issue 4) Either and neither are singular.

When used by themselves (i.e., as pronouns), either and neither are singular.

Writers are often tempted to treat them as plural because they seem to refer to two things.
Beef or lamb? Either is preferable to tofu. (√)
Neither of the sisters is eligible to attend. (√)

(Issue 5) Collective nouns can be singular or plural.

A collective noun is a word that represents a group (e.g., board, team, jury). A collective noun can be singular or plural depending on the sense of the sentence.

The jury is late returning to the courtroom. (√)
(When a collective noun is considered as one unit, treat it as singular.)

The jury are all wearing different colored shirts. (√)
(When the focus is on the individuals in the group, treat your collective noun as plural.)

Often, it’s difficult to make a decision on whether to opt for singular or plural.

A good trick is to precede your collective noun with words like members of, which forces you to go plural.

The members of the jury are late returning to the courtroom. (√)

(Issue 6) Some words that look plural aren’t, and some words that are plural in Latin aren’t in English.

The words listed below often cause issues with subject-verb agreement:
Word Singular or Plural?
agenda Singular
(even though it is the plural of agendum)
criteria Plural
(Unlike data and agendum, criteria has retained its plural status because the singular criterion is still in common usage.)
data Singular nowadays
(even though it is the plural of datum)
measles Singular
media Singular or Plural
(Treat media like a collective noun as opposed to the plural of medium.)
news Singular
Plural only words like glasses, pliers, scissors, trousers, underpants Plural but note that “a pair of [insert word]” is singular.
There is more on this topic in the entry for number.

(Issue 7) The expression more than one is singular.

Somewhat counterintuitively (given its meaning), more than one is singular.
More than one person was involved in this robbery. (√)

(Issue 8) None can be singular or plural.

The indefinite pronoun none can be singular or plural. However, be aware that treating none as plural might irk some of your readers as many people believe none can only be singular.
None of the team is ready. (√)
(This is the safest option, and, let’s face it, it sounds more highbrow.)
None of the team are ready. (√)
(If going singular with none sounds too highbrow for you, you are safe to go plural these days. Hey, live on the edge.)

Here’s a tip: If your none translates best as “not one of,” then treat it as singular. If it translates best as “not any of,” then treat it as plural. If this doesn’t work for your example, then try to treat it as singular. If treating it as singular grates on your ear too much, be brave and go for plural.

If you’re facing the “his/their dilemma” (see also Issue 12), then treat none as plural.
None of the team has polished their boots.
None of the team have polished their boots. (√) (This is far tidier.)

(Issue 9) Terms like half of, the majority of, and a percentage of can be singular or plural.

Expressions such as half of, a part of, a percentage of, a proportion of, and a majority of are singular when they refer to something singular but plural when they refer to something plural.
The majority of my blood is Asian. (√) (Golfer Tiger Woods)
Half of my employees are women. (√) (Businesswoman Christie Hefner)
Seventy percent of success in life is showing up. (√) (Actor Woody Allen)
If eighty percent of your sales come from twenty percent of all of your items, just carry those twenty percent. (√) (US politician Henry Kissinger)

(Issue 10) Number of is plural…most of the time.

The term number of will nearly always be plural.
Lee, a number of cakes have been stolen from the buffet. (√)
A good number of my friends are married, which seems very old-fashioned. (√) (Actress Allison Williams)
Be aware though that number can be singular when referring to an arithmetical value.
The number of women was sixty-four. (√)
The number of women were sixty-four. (X)

(Issue 11) Words like all and some can be singular or plural.

All, any, more, most, and some (types of indefinite pronoun) are singular when they refer to something singular but plural when they refer to something plural.
All of the bread has been stolen. (√)
All of the biscuits have been stolen. (√)
All of Scottish cuisine is based on a dare. (√)
Some of the worst mistakes of my life have been haircuts. (√)

(Issue 12) There’s no suitable possessive determiner to agree with words like someone, and anyone.

Anyone, each, everyone, no one, nobody, and someone are singular. (These words are types of indefinite pronoun.)
The supreme irony of life is that hardly anyone gets out of it alive. (√) (Author Robert Heinlein)
Nobody is ever met at the airport when beginning a new adventure. (√) (Film producer Elizabeth Warnock Fernea)
That all seems pretty straightforward. However, if you use a word like his and her (called possessive determiners) later in the same sentence, problems start to arise.
Anyone who goes to a psychiatrist should have his head examined.
What if the person isn’t male? Here are two good options:

(Option 1) Reword your sentence to make it all plural.
People who go to a psychiatrist should have their heads examined. (√)
(Option 2) Use their instead of his.
Anyone who goes to a psychiatrist should have their head examined. (√)
The English language doesn’t have a gender-neutral singular pronoun for people. This flaw has compelled us to treat their as singular as well as plural.
Read more about treating they and their as singular.

(Issue 13) The positive element governs the verb.

When a subject has a positive element and a negative element, make your verb agree with the positive one.
The CEO not the board members makes the final decision. (√)
(The CEO is the positive element. The board members is the negative element.)
The prawns not the fish were responsible for the vomiting outbreak. (√)
(The positive element is the prawns. The negative element is the fish.)

(Issue 14) Each is singular, but its modifiers often confuse writers.

The word each is often used in a prepositional phrase (e.g., each of the cars, each of the boxes). If this prepositional phrase is the subject, don’t be fooled by the plural modifiers (cars and boxes). The word each is the subject, and the verb must be singular. For example:
Each of the guide dogs is assigned a trainer. (√)

(Issue 15) The subject is plural, but the concept is singular.

It’s not that common, but there are times when the subject and verb don’t have to agree. Look at this example:
Alpacas in a field is a fairly common sight these days.
(This is correct even though alpacas (the simple subject) is clearly plural. Here, our subject is a concept, which is singular.)
Leaving a list of passwords, increasing your life insurance and writing a will, gives you peace of mind while you are on operations.
(Here, we have a compound subject that looks plural, but if you envisage this list of tasks as a singular concept (perhaps under an imaginary heading like “sorting your life out”), then it is possible to use a singular verb.)
(You might also have noticed that the subject ends with a comma. This is not a popular practice (and it will definitely annoy some people), but a comma can be used to end a complex compound subject to group it neatly for your readers.)