You use thank you or, in more informal English, thanks to express your gratitude when someone does something for you or gives you what you want.

You use thank you or, in more informal English, thanks to politely accept or refuse something that has just been offered to you.

You use thank you or, in more informal English, thanks to politely acknowledge what someone has said to you, especially when they have answered your question or said something nice to you.

You use thank you or thank you very much in order to say firmly that you do not want someone's help or to tell them that you do not like the way that they are behaving towards you.

When you thank someone for something, you express your gratitude to them for it.

When you express your thanks to someone, you express your gratitude to them for something.

When people give thanks, they thank God for something good that has happened.

You say 'Thank God', 'Thank Goodness', or 'Thank heavens' when you are very relieved about something.

If you say that you have someone to thank for something, you mean that you are grateful to them because they caused it to happen.

If you say that something happens thanks to a particular person or thing, you mean that they are responsible for it happening or caused it to happen.

If you say that something happens no thanks to a particular person or thing, you mean that they did not help it to happen, or that it happened in spite of them.