A. If you don't stop talking, you won't understand the lesson.

B. If you don't stop talking, you wouldn't understand the lesson.

C. If you hadn't stopped talking, you wouldn't understand the lesson.

D. If you hadn't stopped talking, you wouldn't understood the lesson.

A. If  I didn't have enough money with me now, I would buy that coat.

B. If I had enough money with me now, I would buy that coat

C. If I had enough money with me now, I wouldn't buy that coat.

D. If I didn't have enough money now, I wouldn't buy that coat.

A. Unless you are careful, you will cut yourself with that knife.

B. If you are careful, you will cut yourself with that knife.

C. Unless you were not careful, you will cut yourself with that knife.

D. Unless you were careful, you will cut yourself with that knife.

A. If today were Sunday, the pupils could go swimming.

B. If today is Sunday, the pupils could go swimming.

C. The pupils could go swimming unless today is Sunday.

D. The pupils could not go swimming if today isn't Sunday.

A. safe 

B. sustainable 

C. natural

D. environmental

A. If he left immediately, I  wouldn't call a policeman.

B. If he leaves immediately, I  won't call a policeman.

C. Unless he leaves immediately, I will call a policeman.

D. Unless he leaves immediately, I won't call a policeman.