Both ‘tell’ and ‘say’ refer to the act of communicating verbally with someone, but they are used differently. It is important to keep the differences in their usage in mind, for the two words are not interchangeable.

With direct speech, we use ‘say’. ‘Tell’ is only used with direct speech that is commanding or informative.

She said, ‘I must leave.’

‘Let me go,’ I said.

He told her: ‘Leave.’

‘It is that way,’ he told her.

With reported speech, either of the two words may be used, but as you may have noticed above, ‘tell’ is followed by a pronoun. The object must be specified when using ‘tell’. Thus, you say something, but you tell someone something.

She said that she had to leave.

He told her to leave.