Learn English – What are the differences between “valley”, “gorge”, “ravine”, “canyon”, and “gully”

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."

You lie yourself and you lay something - the very basic thing.

For instance,

You lie down on the floor OVER
You lay a pot on the floor.

This is about the present tense.

I could not find anything better than simply putting GrammarGirl's excellent examples here, in-situ.

The past tense of lie is lay, so...

Last week, Steve lay down on the floor.
The cat lay in the mud after it rained yesterday.

The past tense of lay is laid, so

Last week, I laid the TPS report on your desk.
Mary forcefully laid her ring on the table.

The past participle of lie is lain (this is wonderful), so

Steve has lain on the floor for days.
The cat has lain in the mud for hours.

The past participle of lay is laid, so

I have laid the TPS report on your desk.
Mary has forcefully laid her ring on the table.