Speak

• She can speak French.
• He speaks without accent.
• They speak very quickly and I cannot understand anything.

Talk- when two or more persons have a conversation.

• They are talking about their friends.
• Some uncivilized people talk during the concert time.
• They have just talked about buying a house.

Say

• Don't say anybody we are going to buy a car.
• The teacher said: 'Sit down, please'.
• He said he played tennis for three hours yesterday.

• What did he say?

Tell - used to give information.

• Thank you for telling me what happened.
• As a god mother, she has told fairy tales to their children.
• He told us it would be a rainy day.