Question

1. It's very noisy here. 2. I can't stand it anymore.
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What does 'it' refer to in #1? What does 'it' refer to in #2? Are both refer to the same thing?

Answer 1

In #1, "It" is being used like "there" is sometimes used -- simply to introduce a sentence. Scroll down on both these webpages to see the explanations/examples for "it" --
http://learnenglishteens.britishcouncil.org/grammar-vocabulary/grammar-videos/there-there-are-and-it
and
http://learnenglish.britishcouncil.org/en/english-grammar/pronouns/it-and-there

In #2, "it" refers to the noisy surroundings in sentence #1.