A1
です — being, and being polite
How です works in Japanese, why it goes at the end, and why its negative and past forms look nothing like it.
です closes a sentence and links the topic to what is said about it. It comes last, because in Japanese everything comes before the verb.
私は学生です。
Watashi wa gakusei desu.
I’m a student.
これは本です。
Kore wa hon desu.
This is a book.
It is politeness as much as meaning
です is the polite form; the plain form is だ. Between friends だ is often dropped altogether, and the sentence stands on the noun alone.
彼は学生だ。
Kare wa gakusei da.
He’s a student.
明日、雨？
Ashita, ame?
Rain tomorrow?
Which form you use is not a stylistic choice in Japanese the way it is in English. It encodes your relationship to the person you are speaking to, and getting it wrong is more noticeable than a grammatical slip.
Negative and past
The forms do not look like です, which is the first sign that Japanese conjugates by replacing endings rather than by adding words.
学生じゃないです。
Gakusei ja nai desu.
I’m not a student.
学生でした。
Gakusei deshita.
I was a student.
学生じゃなかったです。
Gakusei ja nakatta desu.
I wasn’t a student.
じゃない contracts ではない, and ではありません is the formal written version. All three are correct; じゃないです is what you will hear.
It does not attach to verbs
This is the trap. です is for nouns and な-adjectives. A verb carries its own politeness in ます, so adding です on top is doubling up.
食べますです。
食べます。
食べます is already polite. です has nothing left to do, and the sentence sounds like someone reaching for politeness twice.
い-adjectives are the exception that confuses people: 寒いです is correct, and there です is adding politeness to a word that was already a complete sentence on its own — see adjectives.