What is raining? In Chinese, I can say "Today is raining, or sky is raining." In English, we can say "it's raining", then what is "it"?

Answer 1

English is an SVO language, meaning each sentence needs a subject (S), then a verb (V), and if available, it can also have an object (O). But the two things each sentence must have are a subject and a verb. So if I said "Is raining", then I would only have a verb (raining) and an auxiliary verb (is), but no subject. In these cases, we use the impersonal "it" as a subject => "It is raining". The same applies for sentences like "It is hot", "It is cloudy", or "It is late".

Answer 2

It is the non-referential subject; it serves simply as a "dummy" stand-in for a condition of being. It is the same as saying "the sky is raining," except for it has more of the sense of the state of rain happening, rather than any specific thing raining on anything else.