The basic steps when constructing a correct sentence where the Subject and the Verb Agree are :
1 – Identify your subject.
For example, Joe’s sister’s dog is really friendly. In this case, the subject is not Joe or his sister. It’s the dog we are talking about.
2 – Is your subject singular, plural or collective?
Singular: The dog is friendly.
Plural: The dogs are friendly.
Collective: Class 5A is waiting for the teacher to enter.
3 – Use the right verb form (or conjugation) for your Subject.
I start, you start, he/she starts, they start, we start, it starts.
Let’s look at collective nouns closer.
Collective Nouns are ways of talking about many things as ONE. For example, a fleet of buses, a box of crayons, collection of stamps, choir, herd, tribe etc. Maybe 1 fleet has 20 buses, 1 box has 20 pencils and a collection has 20 stamps. A choir has many people and a herd contains many goats, for example.
Everyone, Everybody, No one
In the examples above, we are referring to a group of people with the same attitude, desires and actions. So, we can consider them as a singular entity,
In the case of ”police”, you can treat the word as plural or singular.