не было, нет, и/а не будет

[wolfie-18.livejournal.com]

When is it appropriate to use не было, нет, and не будет? Would you say "Экзамены не было трудных" or just "Экзамены были нетрудные?" I'm aware that using these three would mean using the genetive, but I dunno whether or not to use (or if it is allowed) those terms for plural. The same thing with "У меня." Would it be "У меня нет (не есть) машины?"

And another thing, I'm still confused when to use "и/а, но/а." I tried looking at the previous post (in the archive on this topic) but it ended up confusing me more. Thanks in advance.