To me - the difference is whether the phrase with 'the time' is at the beginning or end of the sentence. It's the first time I have been to Moscow. I went to Moscow for the first time in 1980. It's the first time you have asked a question. although: For the first time in a while, I don't know the answer. In that case - it doesn't fit my explanation, although the example is correct, in both structure and meaning! Perhaps the comma changes the grammar? Sorry - I don't know much formal grammar...

The key difference is in the tense of the verb used. Let's look at some examples. 1. He went to Paris for the first time in 1925. (simple present tense) 2. This was the first time he had been to Paris. (past perfect tense)