22a He studies
22b They study.
This kind of agreement is not intrinsically difficult to deal with, but it is possible to lose track of the subject of a sentence or to get confused as to whether the subject is singular or plural. Most people are inclined to say
23a Neither of us wish to go,
23b Neither of us wishes to go,
though when we think about it we know that "neither" should logically be construed as a singular pronoun form, which therefore requires a singular verb form. Once again it is the immediately preceding plural form "us" that throws us off.
The UVic Writer's Guide's advice about writing clear sentences will also give you information about subject/verb agreement that you'll find helpful.