Most roots need a grammatical ending to serve as a word. In Esperanto, the verb does not depend on whether the subject is singular or plural. Studento skribis leteron. A student wrote a letter. Studentoj skribas leterojn.
Students write letters.
Do NOT say the sub in substantivo like the sub in submarine. It's universitato, not juniversitato, in Esperanto. Grammar and sentences. Esperanto has a flexible word order because the ending -n indicates the object of the verb. Studento skribis la leteron.