Think of "tell" and you think of speaking, but "tell" doesn't always mean "tell."

Let me tell you why.

OK, that time I meant speaking.

Not only does the verb 'tell' mean "say something to someone," it also has a meaning similar to "know", "recognize", "understand", or "perceive."

We often use it in combination with the verb "can" to make "can tell" for the present and "could tell" for the past.

I can tell he's from France.

He has an accent!

Or we could tell it was going to rain because of the clouds.

We often use it to talk about differences.

Then, we might use the negative or the question.

Can you tell the difference between this cup and that cup?

I can't tell the difference between this cup and that cup.

We often use "can tell" with the pronoun "you" to talk about something that many people should find obvious.

You can tell he's an English teacher.

He knows all the answers.