Students of English as a second or foreign language often have trouble deciding whether to use say, tell, talk, or speak. This difficulty is not surprising since all four verbs refer to the same activity, and have very similar definitions. The difference between these verbs is found in the patterns in which they are used.
Say: most frequently used to introduce either a direct or indirect quote and always requires a direct object.
Direct Quote (A direct quote is when one person repeats what another person has said.)"The teacher said, 'I have lost my car keys.'"
"The teacher said that he had lost his keys."
Notice that when using said with an indirect quote, said is followed by that.
Tell: is never used when quoting what someone has said. It is more often used to paraphrase what someone has said and always requires an indirect object.
The teacher told his students that he had lost his keys.
There are also certain idiomatic expressions that use tell. Some of these expressions do not require an indirect object while others do.
Idioms using tell include:
tell a story
tell the truth
tell a secret
tell a lie
Talk: usually refers to a conversation between two people. It sometimes requires an indirect object when followed by a preposition.
Students should not talk to one another during an exam.
We are talking about the weather.
Incorrect We are talking about.
About is a preposition and requires an object. We are talking about + noun.
Speak: can sometimes be used in place of talk, but is generally reserved for use in more formal situations.
Did you talk to her about her job performance?
Did you speak to her about her job performance?
Both of the examples above have the same meaning, but in casual speech, native speakers of English are more likely to use talk.
However, in more formal situations, speak would be used.
The new professor has been invited to speak to the faculty about the proposed curriculum.
Speak is also used to greet someone or say hello to someone. When used in this manner, it is always followed by to plus an indirect object.
I always speak to my students when I meet them on campus.
I always greet my students when I meet them on campus.
I always say hello to my students when I meet them on campus.
All three of the above examples have the same meaning.
Speak is also always used with the names of languages.
Yuki speaks English.
NOT Yuki says English.
See also Talking vs. Talking about.