Does the verb 'rain' belong to some special class of verbs since its subject is always 'it'?
Solution 1:
Yes, it belongs to a special "Zero-argument" class of verb.
Normally predicates take 1, 2, or 3 arguments;
respectively, these are called Intransitive, Transitive, and Bitransitive:
- Bill arrived. ARRIVE (BILL) Subject only
- Sarah greeted Bill. GREET (SARAH, BILL) Subject and Direct Object
- Bill gave Sarah a gift. GIVE (BILL, GIFT, SARAH) Subject, Direct, and Indirect Objects
But not all predicates have arguments. This is rare, but weather predicates are a case in point. Weather just happens, and nothing is implicated culturally in its occurrence, beside the event itself.
- It's raining. RAIN () Dummy it subject.
The dummy subject here is called "Ambient it", to distinguish it from the "Distance it" of
- It's a long way to Tipperary.
and the "Extraposition it" of
- It's difficult for me to understand this.
In languages that aren't as fussy about subjects as English, the verb rain by itself (suitably inflected if necessary) is a complete sentence. In Indonesian, hujan means 'rain', both verb and noun, and "Hujan!" is an ordinary sentence that means, unsurprisingly, 'It's raining!'. Just like the English utterance "Rain!".
Solution 2:
All weather-related verbs work this way, not just rain: consider drizzle, drip, snow, sleet, hail. You can even say “It blew hard and long last night,” or “It clouded up.”
English is not a pro-drop language, so every verb needs a subject; when nothing else is available, it will do.
Solution 3:
It is a good question, but 'rain' has little to do with it other than being an example. It is a common example, so much so that some people refer to the 'weather it'.
It is not exclusive though. We could also say "It is dark outside" or "It is clear that the tea will run out"
The it is being used as a dummy subject. This is basically any it with no referent; another form of dummy subject is there - "There seems to be a hold up"
In some ways this can be seen as a form of passive construction, in that the actor is left unstated, and for the same reasons that some people grumble about passives, they also grumble that dummy subjects make your writing fuzzy and unclear. They say that we should write
"I think that the tea will run out"
or
"David thinks there is a hold up"
but that assumes that we actually know who thinks the tea will run out or that there is a hold up. As with passives, there are times when the actor is just not known.