In English, when we want to express the third person singular form of a verb, we usually add an "-s" at the end of the base form of the verb. This is the case for regular verbs.
For example, when we say "he/she/it flies," we are using the verb "fly" in its third person singular form.
The addition of "-es" or "-s" helps to indicate that the verb is referring to a singular subject. This grammar rule applies to most regular verbs in English. It's important to note that irregular verbs may have different forms in the third person singular.
So, in summary, the third person singular form of the verb "fly" is "flies".
Remember to always pay attention to the conjugation rules of verbs to ensure correct usage in different contexts.