What is the difference between “say” and “tell”?

Question

I really get confused when to use say and when to use tell.
Which is appropriate in the following:

• What did he tell? / What did he say?
• What are you saying? / What are you telling?

Answer 1

The usual basic pattern is tell someone. We can add another complement to the verb, if we want, and use tell someone something:

• Tell Bob.
• Tell Bob (that) we need to leave.

We don't usually use tell something without saying which person we were telling.

• *tell (that) we need to leave. (wrong)

The pattern is the other way round with say. The basic pattern is say something. If we really want to add a person, we can say say something to someone, or say to somebody something. Notice that we need to if we include the person we are saying it to.

• He says (that) we need to leave.
• He said (that) he was leaving to me.
• He said to me (that) he was leaving

But we can't say someone.

• He already said me. (wrong)

The original example sentences

What did he _____ ?

What are you _____ ?

In both of these examples, the questions are about the information that was given, not the people that were spoken to. For this reason we need to use the verb say:

• What did he say?
• What are you saying?

So, the patterns to remember are:

• tell people
• say things

Note: There are a small number of 'things' that we can tell. For example we can tell: stories, jokes, lies, the truth - and other words that mean similar things to these. Notice that if someone said He told us a joke, but we didn't hear the last word in the sentence we could say: What did he tell? There are also different senses of the verb tell. One of these for example is to 'detect' something. This has a different grammar.

Answer 2

In your examples, use:

What did he tell you? / What did he say?
What are you saying? / What are you telling me?

This is a very common error. (Which means that you can easily find a good explanation for it on many websites and in most grammar books. And don't worry. Many of us have this error, too!)

Let me focus on the most important case: when you want to use say or tell to mean that someone utters something (to someone), use only:

• say something (to someone), or
• tell someone something.

And you will be safe.

Of course, there are more uses and more patterns, and even more meanings of the two verbs. But this is the most common case that you will use say/tell.

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The link given by josh61, Say or tell ? - English Grammar Today - Cambridge Dictionaries Online, is already very good. To add to that, here is the "Get It Right!" note on say and tell given by Macmillan Dictionary:

Unlike the verb tell, the verb say is never used with a personal object. If you want to refer to a personal object after say, use the preposition to:

✗ Perhaps people misunderstand what I want to say them in English.
✓ Perhaps people misunderstand what I want to say to them in English.
✗ He wanted to be examined by a civilian doctor after an army doctor had said him that it was serious.
✓ He wanted to be examined by a civilian doctor after an army doctor had said to him that it was serious.

The object of the verb say is usually direct speech or a that-clause which reports what someone has said:

“That’s not true!” she said, but her voice betrayed her.
Climate experts say that by 2100 rainfall levels in some areas may rise to five times what they are today.

Don’t use tell in structures like this:

✗ He told that the Japanese representatives tend to be less confident about speaking English.
✓ He said that the Japanese representatives tend to be less confident about speaking English.

Answer 3

One other point I'd like to add to the existing excellent answers:

Say has a strong implication of speaking the message. This would most commonly be done in person, but a telephone conversation (or online equivalent), or some sort of video or audio recording where you are hearing the person speak, strongly relates to the concept of something being said. You could also use it if someone spoke something to someone else, and you hear about it after the fact.

(There are some situations where a person might be said to have "said something" when they didn't speak it - such as in a letter or email - but this is less common.)

On the other hand, telling someone something can be done through almost any means of communication, and the word "tell" (umm, forgive the pun) tells you nothing about how the message was communicated. I'm telling you this information right now, but I'm not saying it, because I'm typing it, not speaking it. I certainly can tell it to you through speaking, but I could also tell it to you in a letter.