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.
- On 6 Aug 2020, The Guardian carried this story, under the headline Kate Tempest announces they are non-binary, changes name to Kae: "The musician and poet formerly named Kate Tempest has changed their name to Kae Tempest, and announced they are non-binary. In the announcement on Instagram, Tempest said they were changing the pronouns they use, from she and her to they and them. Their new name is pronounced like the letter K. ...'". On 9th August, Neil Ferguson made this witty correction: "Kae Tempest has adopted the non-gendered pronouns they and them [...]. Kae may not be either male or female, but is not more than one person. For a more pleasing grammatical concinnity, it should be 'they is', not 'they are'." (AWE is grateful for the rare word 'concinnity'.)
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.