N5
です
To be, Is
あなたです。
It is you.
Explanation
「です」 also means 'is / am / are', the polite way: use it with teachers, bosses and strangers. It goes after a noun or a な-adjective. It can also go after an い-adjective, but then it only makes the sentence politer and has no 'is' meaning. The negative is 「ではありません」, and in speech 「じゃありません」 or 「じゃないです」.
Mixed up with
After a noun or a な-adjective, 「です」 is the real 'is', and the negative is 「ではありません」.
After an い-adjective, 「です」 only adds politeness; the adjective itself changes for past and negative: 「おいしかったです」, 「おいしくないです」. Never 「おいしいでした」.