Teacher Discussion Forums

Could any of you direct me to a good website or recommend me some other resources where I could refresh my memory on what words can and cannot be omitted when combining ideas? Just and example to illustrate what I mean:

He stares at the coffee cups which are in a pile. + The coffee cups are stained with lipsticks. = He stares at the coffee cups which are in a pile and stained with lipstick.

To me the combined sentence would sound better if "are" was repeated, but I can't really come up with some convincing argument. I need a refresher course, so a good website would be greatly appreciated.

He stares at the pile of lipstick-stained coffee cups.
He stares at the pile of coffee cups stained with lipstick.
He stares at the pile of coffee cups which are stained with lipstick.
He stares at the coffee cups which are in a pile and stained with lipstick.

Sorry, couldn't resist. There are lots of ways to put those sentences together. I don't know the "rule" though, so hopefully someone will be able to direct you. I think, however, that it's a case of "the smaller the sentence the better."