What's the difference between "for your information" and "I tell you"?

Answer 1

I agree that "for your information" is used more formally to convey side or secondary information. "FYI" is used to do the same (but less formally).

For conveying main or primary information, use the phrase "I am writing to inform you" or I would like to inform you that . . . " A bit less formally, but still very politely, you can convey information by starting with the phrases: (1) "Following up on your request, . . . "; (2) "The information I obtained (or gathered or found) is . . . "

We do not generally use the expression "I tell you" to convey information, either in writing or orally.

Answer 2

"For your information" is formal. We would use that at work or in business to give information that is not critical but secondary (or on the side). We often use it at the beginning of a statement to explain that this information may be useful to the reader/hearer. Some people write "FYI" in casual writing. Example: "FYI, there is some fresh coffee in the employee kitchen."

"I tell you" is not used very often. It can be used in informal conversation to emphasize a statement. For example, "It is so cold outside. I tell ya!" (here it means, "it really is cold.") It's like saying something twice to emphasize it.

You might also hear people say, "I'm telling you..." during an argument. For example, "I'm telling you—if you don't stop eating that cake you're going to get sick!" It means, "I really want you to hear this!" This is argumentative speech though. It is not to be used in formal speech or writing. It could be understood as too confrontational or rude.