By has no third person singular form. The monosyllabic form is the third person singular of the predicate verb. Generally speaking, the verb +s and by are prepositions, so there is no monosyllabic form. The third person singular is the pronoun expansion data used in language to indicate people other than the two sides of the conversation. The third person is relative to people other than the speaker (first person) and the listener (second person) in conversation. Singular means that the other person mentioned in this sentence is one person, not two people (even number, widely used in Pacific island indigenous languages) or more (plural number). For example, he, she and it in Chinese, he \ she \ it \ he \ she in English. In English teaching, the third person singular refers not only to this group of pronouns in English, but also to the related grammar of this group of pronouns.