When do you ‘say’ and when do you ‘tell’?

“I said him not to worry”
“He said to me to come tomorrow”
He told me, “No job for you.”
I am saying the truth.
She says good jokes.
The receptionist told, “Good evening.”

When we conduct the mock Speaking Test in the IELTS training course, I have heard students getting confused between the usage of the words, ‘say’ and ‘tell’. So what is the difference between the two? Can they be used interchangeably? Why do some of the sentences listed above not sound correct? It is a question of word choice and usage.

The word ‘said’ is the past tense of the word ‘say’ and the word ‘told’ the past tense of the word ‘tell’.
Perhaps if we remember that we do not need someone to say something but we need someone to tell something, we may avoid the errors in usage. When you are telling, you are telling to someone. You need an object for the verb ‘tell’. You cannot ‘tell’ in thin air. Telling is addressed to someone or something. Whereas a person can say something without it being addressed to someone. Some examples:
Has she told you about her promotion?
Please tell us what is going to happen next.
The doctor told him to take the medicine twice a day.
Can you stop crying and tell me what happened?

In all the above four sentences you can see that there is an object for the verb ‘tell’. The objects are, ‘you’, ‘us’, ‘him’ and ‘me’. If you had replaced ‘told’ with ‘said’ each of the sentences would be awkward.

‘Tell’ can be used in a sentence without an object when less factual information is being shared.
She loves telling silly jokes.
I am not telling what happened.

‘Say’, on the other hand, has to be used when giving the exact words that the person has said with or without quotation marks.
I said, “How long is the journey?”
I went in to say goodbye.
He had said three sentences when the microphone got switched off.
If you notice, there is no object for the verb ‘say’.

Another place ‘say’ is used is when expressing opinions.
They say eating twice a day is healthy.
I wouldn’t say my daughter sings well.

If you replace ‘say’ with ‘tell’ in the above two sentences, it would again be an awkward construction.

I hope you will say this piece has been useful and tell more of your friends about it.