Can the word hard be used when describing rain as in " the rain was hard/ it was hard rain?"
Or is it wrong to use " hard" with rain?

Question

Can the word hard be used when describing rain as in  " the rain was hard/ it was hard rain?"
Or is it wrong to use " hard" with rain?

Answer

You could say:  It was raining cats and dogs. I hear a lot of American use this slang, It is very common slang :)

Answer

You can also say "heavy rain".

For example:

It has been raining heavily all afternoon.

Answer

Usually we say "it is raining hard" or "it was a hard rain." It means that it rained a lot.

Answer

You might be interested in singer Bob Dylan's famous song, "A Hard Rain's a-Gonna Fall." Like many song lyrics it is not strictly correct good English, but it does illustrate the phrase "hard rain."

Wikipedia, http://en.wikipedia.org/wiki/A_Hard_Rain's_a-Gonna_Fall
Lyrics, http://www.bobdylan.com/us/songs/hard-rains-gonna-fall
Youtube, http://www.youtube.com/watch?v=-ex-m-eEKsg

Answer

You have to be careful though, consider this question and 2 different responses:

Q: What's the weather like out?

A1: It is raining hard out there.
A2:  It is hardly raining out there.

Those two answers actually mean completely opposite things.  A1 means there is a lot of rain falling.  A2 means it is not raining very much out.