Learn English – What are the differences between “I don’t mind”, “I don’t care” and “It doesn’t matter”

Using speak, tell, say, and talk is a very common problem for ELL.

@Siddhartha and @Em1 gave great answers, so let me quickly list only those aspects not mentioned by them.

So, I have some meaning to convey. How do I know which word to use?

• Ability — if it's about ability, "talk" is preferred: "cats can't talk", "when I'm drunk I lose my ability to talk". Note that "speak" is allowed but is less popular;
• Languages — use "speak": "I speak English";
• Action — covered in answers above; both words can be used; "speak" is mostly about single-direction, while "talk" usually refers a conversation (and is less formal);
• Express information — use "say" or "tell": "I told her that I love her". Here, "I love her" is the information being expressed. "Say" can also be used, but it connotates a single-time action. Compare: "I said that the discussion is over and hung up the phone."

Consider this sentence:

I can see they are talking, but I don't know what they are saying.

Here, you see an action, but you don't know or don't understand the meaning of the information others are exchanging.

• Wide topic, narrow topic:
  • use "say" or "tell" if the topic is narrow or it's about a small piece of information: "I told her there is a party tonight";
  • use "talk about" or "speak about" when the topic is wide: "We are speaking about history". There's also an idiomatic construct, "talk politics" that omits "about".
• Conversing during an extended period of time: use "talk about" or "speak about", as above;
• Imperative usually follows the rules above, but due to an extended popularity, here's a brief:
  • Extended time or dialogue — "Talk to me."
  • Long monologue — "Tell me about your problem."
  • Short monologue — "Say something!"
  • Requesting a certain attribute of speaking process — "Speak slowly please."

Anytime a person returns communication it can be called a response or a reply, while an answer is a form of response which is a solution to a problem or question. So response and reply are generic and can be used in any situation, while answer is more specific in its usage.

So if you asked a question or asked for a solution to a problem, and the person gave it to you, then you can say "Thanks for your quick answer". If it was not in one of these categories, then use response or reply since these are both generic.

And if you are still in doubt, remember that because response is generic you can use it in any situation.