Language / Help with the Polish plurals and describing of objects? [22]

I have been using a website but sometimes their lessons don't have all of what they ask you to do after it.
What I've been doing at the moment is describing objects and numbers and they want me to write sentences on it.

For example "There are five buildings"

I can say 5 (Pięć) and building (budynek) but how would I write it. I assume the form of budynek changes because that's what was happening in the lesson but I don't understand why? (budynków, budynki?)

And also, would I just use "są" for the 'there are'?

Są pięć budynek(?)

Also "Three buildings are not green"'

'are not green' = nie są zielone

What form would three budynek take?

I've been getting confused with how they used the word for dog and how it changes -
1 dog = Jeden pies
2 dogs = Dwa psy
3 dogs = Trzy psy
5 dogs = Pięć psów

And then the numbers keep changing eg - jeden to jedno, dwa to Dwie, trzy to Trzech etc.

Please help explain all this to me because I'm soo confused!