Fill in the blank with the most appropriate word. I _________ write to you if you don’t give me your address.

The correct answer is: can’t.

The other options are:

• couldn’t – This would be the correct answer if the sentence was in the past tense. For example, “I couldn’t write to you if you didn’t give me your address last week.”
• wouldn’t – This would be the correct answer if the sentence was expressing a negative opinion or intention. For example, “I wouldn’t write to you if you didn’t give me your address because I don’t want to talk to you.”
• didn’t – This would be the correct answer if the sentence was about something that happened in the past. For example, “I didn’t write to you because you didn’t give me your address.”

In this case, the sentence is about something that is possible in the present. Therefore, the correct answer is “can’t.”