1. Why don't you study hard?

• This is an imperative sentence, used to give advice or make a suggestion.

2. Keep silent or you'll wake the baby up.

• This is a conditional sentence of the first type, expressing a real possibility in the present or future.

3. I don't know the answer, so I can't tell you.

• This is a compound sentence, connecting two independent clauses with the coordinating conjunction "so."

4. We got lost because we didn't have the map.

• This is a complex sentence with a subordinate clause of reason introduced by "because."

5. My friend was so late, so he missed the train.

• This is a compound sentence, connecting two independent clauses with the coordinating conjunction "so."

6. This car is too heavy for me to move.

• This is a complex sentence with an adjective clause modifying the noun "car."

7. Morning exercises make me feel healthy.

• This is a simple sentence with a transitive verb.

8. The church bells keep me from sleeping.

• This is a simple sentence with a verb phrase "keep...from."

9. Without air, we would die.

• This is a conditional sentence of the second type, expressing an unreal situation in the present or future.

10. I advise him to stop smoking.

• This is a simple sentence with the verb "advise" followed by an infinitive.

11. Be calm, or you'll make a wrong decision.

• This is a conditional sentence of the first type.

12. Without a visa, he couldn't have come to that new country.

• This is a conditional sentence of the third type, expressing an unreal situation in the past.

13. Today isn't Sunday, so the students don't go swimming.

• This is a compound sentence, connecting two independent clauses with the coordinating conjunction "so."

14. Because you don't leave immediately, I'll call the policeman.

• This is a complex sentence with a subordinate clause of reason introduced by "because."

15. I don't know her number, so I don't ring her up.

• This is a compound sentence, connecting two independent clauses with the coordinating conjunction "so."

16. Go right now or you will be late for the train.

• This is a conditional sentence of the first type.

17. As she is often absent from class, she can't keep pace with her classmates.

• This is a complex sentence with a subordinate clause of reason introduced by "as."

18. I couldn't write to Alice because I didn't know her address.

• This is a complex sentence with a subordinate clause of reason introduced by "because."

19. The heavy rain kept everybody from going outdoors.

• This is a simple sentence with a verb phrase "keep...from."

20. I didn't watch TV last night because I was busy.

• This is a complex sentence with a subordinate clause of reason introduced by "because."

21. People don't see the UFO, so they don't call the National UFO Reporting Center.

• This is a compound sentence, connecting two independent clauses with the coordinating conjunction "so."