14 Modal Verbs 8 Fill in the gaps with mustn't or don't have to. A teacher is speaking to her students before a test: "You are going to have a test today. Your test papers are on your desks. You 1)...mustn't... begin until I say so You 2) time. You 3) You 4) hurry; there is plenty of speak during the test copy your friends' work and you 5) open any of your books. You 6) write in pen; you can write in pencil. You 7)..... can leave but you 8) .... stay when you've finished; you ...... be noisy. When you leave, you 9) wait for your friends outside the classroom. Wait outside the building. Finally, I wish you all good luck. You can begin now!" Must is used only in the present tense. It borrows the rest of its tenses from the verb have to. Have to changes into has to in the third person singular in the present affirmative and into had to in all persons in the past affirmative. Have to takes do/does/did to form its questions or negations and it is followed by a bare infinitive He had to leave early. Did she have to leave early, too? She doesn't have to finish it now. She has to finish it by Monday. I'll water the plants tonight, so you won't have to do it tomorrow.​repedee