Let's analyze the sentences to find the best combinations for the given questions.

### Question 3:
**Sentences:**
1. I don’t study hard. I don’t have good marks.

**Options:**
A. I can have good marks if I study hard.
B. I couldn’t have good marks if I had studied hard.
C. Had I not studied hard, I could have had good marks.
D. If I studied hard, I could have good marks.

**Best Answer:**
D. If I studied hard, I could have good marks.
**Explanation:** This option clearly links studying hard with the possibility of having good marks.

### Question 4:
**Sentences:**
1. She explained it clearly. I could understand it.

**Options:**
A. She didn’t explain it clearly enough for me to understand.
B. She explained it so clearly that I could understand it.
C. I couldn’t understand it because she explained it clearly.
D. Although she explained it clearly, I could understand it.

**Best Answer:**
B. She explained it so clearly that I could understand it.
**Explanation:** This option effectively combines the two sentences by showing the cause (her clear explanation) and the effect (my understanding).

Thus, the answers are:
- **Question 3:** D
- **Question 4:** B