Hmm, it is a good question. In English, sentences are made in order of subject → verb → object, which can be often shown as S → V → O. But it was S → O → V in the past.

You know, both S and O are nouns, so it’s difficult to distinguish which is which when they are lined up.

Next, we have imperative sentences. This rule is also known as the imperative mood, in which you can order or request someone in front of you to do something. You don’t need the subject.

It’s simple. No need to use the subject. When you want to order or request something directly to the people in front of you, you don't have to identify them.

Next I’ll explain the negative sentence. You use the auxiliary do in sentences with general verbs, but can you tell me why?

In English sentences, it is preferable that the verb follows the subject. Therefore, rather than saying “I say not”, “I do not say” could satisfy needs special to native English speakers.

It used to have the same meaning of “let”, a causative verb, but now it’s only used as the code in negative and interrogative sentences.

If you only put “do” at the beginning of a sentence and give it the meaning of interrogation, you don’t have to change the order of the subject followed by the verb.

But, this rule is only true in interrogative sentences with general verbs. If it’s a sentence with a “be” verb, you’ll get the VS pattern in the interrogative sentence, which had existed before the SOV pattern.