Part 1: How to conjugate verbs into the ない-form
Group I: V-i ます → V-a ない (Note: words ending in い change to わ)
Group II: V ます → V ない
Group III: します → しない,
→ turn on -> do not turn on
→ play -> do not play
→ smoke -> do not smoke
→ speak -> do not speak
→ hurry -> do not hurry
→ wait -> do not wait
→ have -> do not have
→ come -> do not come
Part 2: ~Vないでください (Please don't ~)
Meaning: Please don't ~.
Used to ask someone not to do something. This is a softer request than Vてはいけません.
B: はい。わかりました。
→ A: Please don't drink beer from now on. B: Yes, I understand.
B: いいえ、入れないでください。
→ A: Shall I put in sugar? B: No, please don't.
Part 3: Vなくてもいいです (You don't have to ~)
Meaning: You don't have to ~ / It's okay if you don't ~.
→ Because there is time, you don't have to hurry.
→ August is cool, so you don't have to turn on the air conditioner.
Part 4: Vなければなりません (Must / Have to ~)
Meaning: Must ~ / Have to ~.
→ Because there is a test tomorrow, I have to study tonight.
→ Because I am sick, I must take medicine.
Part 5: までに (By / Before ~)
Meaning: By / Before ~.
Indicates a deadline or limit for an action.
B: 金曜日までに だしてください。
→ A: By when must the report be submitted? B: Please submit it by Friday.
B: 10時までに かえらなければなりません。
→ A: By what time must you return to the dormitory? B: I must return by 10 o'clock.
Part 6: Object as the topic
Meaning: When you want to make the direct object of "Nを V" the topic of the sentence, replace the particle を with は and move Nは to the beginning of the sentence.
→ 荷物は ここに 置かないで ください。
→ Please don't put luggage here. → As for the luggage, please don't put it here.
→ 昼ごはんは 会社の 食堂で 食べます。
→ I eat lunch at the company cafeteria. → As for lunch, I eat it at the company cafeteria.