Why “What if I told you…” and not “What if I tell you…”?

Exactly in which category of grammar does this type of sentence fall?

What if I told you …

Why don't we use tell instead of told, as it sounds this event has happened in past. Can anyone explain this in detail?

Solution 1:

Because that is the form we use in English for counterfactuals. Historically it was the past subjunctive, but in Modern English that is distinct from the ordinary past in precisely one case: I/he/she were rather than was. This is why some people say

If I were to tell you, ...

(Others have lost this and say If I was to tell you ...)

Teachers of English as a foreign language refer to this as the second conditional (a phrase I had never heard until I started reading this site).

What if I tell you ...

is the so called first conditional, and refers to an unknown future, rather than a counterfactual. With a first-person subject, there's not too much difference in meaning, but if I tell you is saying it is quite possible that I will tell you, whereas if I told you is considering it as a hypothesis, probably less likely.

See Second conditional in Wikipedia for more information.