1. Поставьте предложение в отрицательную форму: I was able to answer the question at once.
1. I wasn't able to answer the question at once.
2. I wasn't able answer the question at once.
3. I wasn't to be able to answer the question at once.
4. I wasn't to be able answer the question at once.

2. Поставьте предложение в отрицательную форму: They must have studied the contract terms.
1. They can't have studied the contract terms.
2. They must have not studied the contract terms.
3. They must not have studied the contract terms.

3. Поставьте предложение в отрицательную форму: You should have followed their advice.
1.You should have not followed their advice.
2.You shouldn't have followed their advice.
3.You didn't should have followed their advice.
4.You shouldn't haven't followed their advice.