1.It is possible that he read it in the paper 2. You didn't thank him for his present 3. I opened it, which was unnecessary 4. I gave him a tip, which was not necessary

1. He can read it in the paper.

2. You should have thanked him for his present.

3. I didn't need to open it.

4. I didn't have to give him a tip.

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1. S + be + possible = can: có thể

2. should have + V3/ed: đáng lẽ nên làm điều gì trong quá khứ nhưng lại không làm

3. unnecessary = not need to V: không cần thiết phải làm gì (=not have to V)

4. not necessary = not have to V: không cần thiết phải làm gì (= not need to V)