There are many kinds of subject -verb agreement :
the general rules and the special expressions which of the two will i answer?
In the general rules
-the singular subjects always takes a singular verb or SS=SV
and the plural subjects always takes a plural verb
ex.the list of the items (is,are) on the desk.
the special expressions has very many examples but i can still give you:
ex.A number of students (is,are) absent.
the expression (the number) always takes the singular verb and the expression (a number) always takes a plural verb!,..