I don't think I did something or I don't think I do something or I think I didn't do something?

Question

I want to say that I did something a few months ago and I don't remember whether I've done that.

Which one should I say?

1. I don't think I did something.
2. I don't think I do something.
3. I think I didn't do something.

Answer 1

Firstly, I think you already know this, but you should replace something with a more specific word or phrase:

1. I don't think I did that.

Let's look at each sentence in turn:

1. I don't think I did that.

I don't think I left the door unlocked [a few months ago].

This is the most common way to express this thought. "I don't think" is present tense and "I did that" is past tense.

2. I don't think I do that.

I don't think I leave the door unlocked.

"I don't think" is still present tense and now "I did that" is present tense, which indicates habitual action.

3. I think I didn't do that.

I think I didn't leave the door unlocked [a few months ago].

This could be either a once-off event, or habitual action in the past. If it is interpreted as habitual action, it implies that it used to be that case but it has changed since:

I think I didn't leave the door unlocked [a few months ago], but I think I do leave the door unlocked now.

So your best choice is #1, followed by #3 but #1 is much clearer than #3. #2 means something different from what you want to say.