grapefruit(n) 葡萄柚
I love to eat grapefruit.
Grapefruit is sweet.
Some grapefruit is sour.
Good!

solemnly (adv) 郑重
He speak solemnly.
{He} is the third singular, so please add an -s at the end of {speak}.
He speaks solemnly.

He writ solemnly.
You mean {write}? Also, remember to use the third singular form.
He writes solemnly.

The school make a solemnly rule.
{Solemnly} is an adverb which can’t be used to modify a noun.
{The school} is the third singular, so please add an -s at the end of {make}.
The school makes a rule solemnly.