Learn English – “There is no SINGULAR NOUN” and “There are no PLURAL NOUN”

The word "police" is rather special: It has no singular noun form. Something like that police over there is securing the scene would be incorrect. One would always construct sentences in the plural form like so:

The police are out in force today.

Anything done by the police will reflect on them.

Other words that take no singular form would include pants, trousers, scissors, and clothes.

Confusion arises because "police" is also used as an adjective. Consider these sentences:

A police department is housed in that building.

The police chief was highly visible at the town meeting.

In these two sentences, we are not speaking of "a police". You could easily remove the word from both sentences and they would make sense semantically and grammatically. Instead, the word describes the department or chief. It gives us context.

"Police" also has a verb form. You may encounter it like this:

The Boy Scout troop must police the area before they leave to remove any trash.

The verb means "to investigate, to search, to clean up". This certainly does fit in with a subset of the duties of a police department.

When we are looking at a sentence with words that indicate portions (percent in your example, also: fraction, some...), to choose the correct subject-verb agreement, we simply have to look at the noun after "of".

In the first case, the noun is "students" - a countable noun and it is in plural, therefore the verb should be in plural as well:

20% of the students are present

Even if there are 100 students, and you want to infer 1%, you still use plural:

1% of 100 students are present

In the second case, the noun is "protein" - countable noun in singular, the verb is singular as well:

The remaining 20% of the protein forms enzymes