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How to use and say "a massive thank you?" i mean under what conditions we can use the word 'a massive thank you'? and what's the difference between thank you and a massive thank you?
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A massive thank you means a very, very big thank you.

Use it rarely. It's rather informal.
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Under what conditions we can use the phrase 'a massive thank you'? and what's the difference between thank you and a massive thank you?

It's a matter of degree.

You say 'Thank you' when somebody holds a door open for you.

You might say (though I wouldn't) 'A massive thank you' when somebody gives you £100,000 to prevent a loan shark breaking your legs and arms for not repaying a debt.