ref – https://www.grammarly.com/blog/subordinating-conjunctions/

A subordinating conjunction is a word or phrase that links a dependent clause to an independent clause. This word or phrase indicates that a clause has informative value to add to the sentence’s main idea, signaling a cause-and-effect relationship or a shift in time and place between the two clauses.

For example, ‘because’.

Because is a conjunction with just one purpose: to show a cause-and-effect relationship between a subordinate clause and a main clause. On its own, a clause beginning with because is incomplete.

Because he wouldn’t wear a seat belt.

We have the sense that there is something missing here. Let’s add an independent clause so this statement has something to lean on.

Independent clause:

Robin wasn’t allowed in the Batmobile any longer.

Now we will combine the two in a complex sentence.

Robin wasn’t allowed in the Batmobile any longer.
He wouldn’t wear a seat belt

[Robin wasn’t allowed in the Batmobile any longer] because [he wouldn’t wear a seat belt].

because links them together. The first clause explains the effect. The 2nd clause, the ’cause’.

Other subordinate conjunctions include:

for, as, since, though, due to, provided that, because of, unless, and so/so that.

Example:

subordinate conjunction: hence

ind clause – Batman required strict compliance with seat belt rules
ind clause – Robin was not allowed to ride in the Batmobile

[Batman required strict compliance with seat belt rules], hence [Robin was not allowed to ride in the Batmobile].

Example:

sub onjunction – Since
ind clause – Robin refused to wear his seat belt
ind clause – Batman has banned him from the Batmobile

Since Robin refused to wear his seat belt, Batman has banned him from the Batmobile.

Subordinating conjunctions signaling relationships of time or place

Another function of subordinating conjunctions is to show a relationship between two clauses involving a transition of time or place. Some examples of such subordinating conjunctions are once, while, when, whenever, where, wherever, before, and after.

sub conj – Once
ind clause – Batman learned that Robin had not been wearing his seat belt
ind clause – he took away his keys to the Batmobile

Once Batman learned that Robin had not been wearing his seat belt, he took away his keys to the Batmobile.

subj conjunction – whenever
ind clause – Robin looked regretfully at the Batmobile
ind clause – he passed it in the Batcave.

Robin looked regretfully at the Batmobile whenever he passed it in the Batcave.

subj conjunction – After
ind clause – Batman was done working for the night
ind clause – Robin took a secret ride in the Batmobile

After Batman was done working for the night, Robin took a secret ride in the Batmobile.

subj conjunction – Before
ind clause – he must promise to stop playing with the Batmobile
ind clause – Robin gets his job in the Batcave back
Before Robin gets his job in the Batcave back, he must promise to stop playing with the Batmobile.

Comma placement and subordinating conjunctions

Subordinating conjunctions that fall in the middle of a sentence are generally not preceded by a comma.

Here, we have subordinate conjunction ( whenever ) falls in the middle of the sentence:

[Robin drove the Batmobile] whenever [Batman was away].

However, when a subordinate clause begins a sentence, the whole clause (but not the subordinating conjunction itself) is followed by a comma.

Whenever Batman was away, Robin drove the Batmobile.

after
although
as
as if
as long as
as much as
as soon as
as though
because
before
by the time
even if
even though
if
in case
in order that
in the event that
lest
now that
once
only
only if
provided that
since
so
supposing
than
that
though
till
unless
until
when
whenever
where
whereas
wherever
whether or not
while