Here are the completed sentences using the correct forms of the verbs in brackets:

1. Stop talking or you won’t understand the lesson.
=> If you **don’t stop** talking, you **won’t understand** the lesson.

2. I don’t know the answer, so I can’t tell you.
=> If I **knew** the answer, I **could tell** you.

3. We got lost because we didn’t have a map.
=> If we **had had** a map, we **wouldn’t have got** lost.

4. Without this treatment, the patient would have died.
=> If the patient **had not received** this treatment, he **would have died**.