Agreement of subject and verb (number)
Words (particularly Subject and Verb, in English) should agree in number. That is, if you are talking about one person in the present tense, you should use the verb-form ending in -s, not the basic verb form, for example "Sally goes", not "
Sally go"; "John says", not " John say".
In the same way, if you are talking about more than one person, call them they, not he or she - and the other way round, for example "Students may think that they can write well", and "A student may think that he [or she] can write well". Do not write " A student may think that they write well".
Note that it is probably easier - certainly more elegant, in AWE's opinion - to use the plural form: the first of these examples.
Note that formal, academic English sometimes appears to use a plural verb where ordinary spoken English doesn't. When we use the word if (and others like it) to talk about imaginary and indeed unlikely, if not impossible, situations, we often use different forms of the verb. (In other European languages, this use of 'if' often demands the subjunctive; but unless you are aware of it, such grammatical theory doesn't seem to help native speakers.) Examples include "If I were you..." (I can't possibly be you); "If that were the case..." (this implies that I think this is not so); and "If that be the case..." (this is like the previous example, only slightly weaker: "It may be so" (but I doubt it').
See also agreement in grammar.