rain (noun) is condensed water vapor falling from clouds
the plural is rain, unless you are speaking of several rain episodes
The spring rains caused much flooding.

to rain (verb) is to have rain fall from the sky

It is conjugated as follows:
it rains
it rained
it will rain
it has rained
it had rained
it will have rained
it would rain
it would have rained

snow is grammatically identical

For more information, visit Everything Language and Grammar.