です (des(u)) – it is, present positive
でした (desh(i)ta) – it was, past positive
じゃありません (jaarimasen) – it is not, present negative
じゃありませんでした (jaarimasendesh(i)ta) – it was not, past negative
Sentence order:
です comes at the end of a sentence.
Noun + です = Noun, it is
山です = Yama desu = Mountan, it is = It is a mountain