“You must help me prepare lunch” is a declarative sentence. A declarative sentence is the type of sentence used most often in English. It is usually a statement and more often than not the subject is followed by a verb.
Even though the sentence appears to be an imperative sentence, imperative sentences very rarely use a subject in the sentence.
If the sentence was help me prepare lunch then we can say it is an imperative sentence as it is giving a command and begins with a verb which is common with imperative sentences.
Below are sentences that may look like imperative sentences but are actually declarative, notice the use of modal verbs in these sentences:
- You should help me prepare dinner
- You have to do your homework.
- You may go to the bathroom.
- You must complete the exam.
- You ought to go to the new play.