Is it necessary to add s or es after the verb Susan can't speak English? 2. Say is going to type the boss's 3. His sister can cook for the family

Is it necessary to add s or es after the verb Susan can't speak English? 2. Say is going to type the boss's 3. His sister can cook for the family

First of all, after the third person singular, the verb should follow the subject with the singular (add s or ES), but if there is a modal verb, the verb does not need to add S. the latter es modal verbs include can, could, shall, should, may, might, out to, etc

Does the predicate verb of who's special question use the third person singular? Why?

Who may be followed by singular or plural predicates
who is the man that stands there?
Who are the people that are present the meeting ?

Why is "who" followed by the singular third person form in special questions?

Eg: who is that woman
who are your parents
The key is to see whether the object is singular or plural