"It was the first time I did" or "It was the first time I do"?

Question

I did something in the past. Now, when I mention it to others, should I say:

It was the first time I did that.

or:

It was the first time I do that.

Which part of English grammar should I have a look to understand it?

Answer 1

To be totally correct, you should actually say,

It was the first time I had done ____

which is the past perfect tense. However, you may also hear the simple past used, which is the first of your sentences.