We use say when we report someone’s words.
She said ‘This is horrible!’
He said that he wanted a drink.
We use say when we ask about language.
A: How do you say ‘book’ in Spanish?
We say hello / goodbye please / thank you
Happy Birthday / Merry Christmas / Happy New Year / Congratulations
Tell is usually followed immediately by a person. Say is not followed immediately by a person.
He told me his name. [NOT He said me his name.]
We use tell when we want to know how to get to a place.
Can you tell me where the bus station is, please? [NOT Can you -say me …?]
We use tell with other wh-words too (when, how, why, where), e.g. you can tell someone how to do something, where something is, why something happened.
He told me how to send a fax. Tell me when you want to have dinner.
You can tell someone the time / a story / a joke / your name / address / telephone number.
Ask is used for questions.
My sister asked me where I was going, (or My sister asked (me) ‘Where are you going?’)
A: Can I ask you a question?
A: What day of the week were you born?
You can ask someone the way / the time / a question.
Ask somebody to do something and ask someone for something
I asked him to turn off his radio, (or I said ‘Please turn off your radio.’)
She asked for the bill, (or She said ‘Can I have the bill please?’)
Do you speak Japanese? [not Do you -talk Japanese?]
I like talking to you. (having a conversation with you)
Can you answer the telephone / the door, please?
(pick up the phone / open the door to see who it is)
I wrote a letter to him but he did not reply.
(for letters/faxes/e-mails) (he did not send me a letter back).