Do sentences in English always have a subject?
Yes. All sentences in English have a subject, although in some
cases the subject is implied/understood.
Examples-
declarative: "You kiss boys."
interrogative: "You kiss boys?" or "Do you kiss boys?"
exclamatory: "You kiss boys!"
command: "Kiss boys."
In all of these, the subject is 'you'; 'kiss' is the verb and
'boys' is a direct object ('do' is a helping verb, so when used, it
is part of the verb form). Even though there is no 'you' in the
command sentence, it is understood that the subject is the person
to whom the sentence is spoken, which is... you.