I learnt at school that you use present-perfect form + ever to express your experience in cases such as "This is the first time I have ever eaten sushi."
My question is whether you ever say "This is the first time I eat sushi" or "This is the first time I ate sushi".
Do you always need to use present-perfect form + ever with "This is the first time"?
I can imagine the first as a dramatic introduction before eating the sushi (possibly holding one piece in your hand). However, the sentence is somewhat awkward (and possibly not 100% grammatical, although definitely understandable in that context). It would make much more sense with "this is going to be" or "this will be".
The second makes no sense: if you ate the sushi already, then it "was" the first time you ate sushi, not "is".
"This is the first time I eat sushi" means you are actually eating it at this very moment. "This is" here is deictic. It's similar to: This is an apple. You are referring to the event of eating sushi.
If you have ** eaten it**, you can say "This is the first time I have eaten sushi" because in fact, it was in the past even though it may be five minutes ago. So, you want to signal that it is past, but not WHEN: two minutes ago or even one minute ago. It is in your stomach, so it is a past thing. And it's not: I ate sushi one minute ago.
And: That was the first time I ate sushi. You can't have "This is" with a past event.
This is takes place in the present time.
