Mean vs think

in my language these two words mean the same. I understand where to use these words but I am not able to explain it to my friend? does it have a rule?

just to be sure:
guy a: I think that the girl over there is pretty.
guy b: Do you mean the left one?

I’m really sorry but in our language these two words have got really the same meaning.

Does it have a rule where to use it or in English it mean two different words and there is no doubt where to use mean and where think?

Thank you very much

a: ‘think’ here means ‘regard’. ‘I think’ is always used to suggests a personal opinion of the speaker.
b: ‘mean’ here means ‘refer to specifically’. ‘Guy b’ trys to confirm if the pretty girl ‘guy a’ said is the left one between, say, the two girls.

So, they are different.

thank you very much