Grammar Point
polite (teineigo) language
pol 
copula
cop 
です

be, is

Noun or な-Adjective

The polite copula used to declare that something is or was a certain way in formal or polite situations. It acts as the polite equivalent of the plain copula だ.

Noun + です / でした

Used with nouns to politely identify what someone or something is or was.

I am a student.

It was rainy yesterday.

な-Adjective stem + です / でした

Used with the stem of な-adjectives to politely describe a state or quality.

This room is quiet.

He was kind.

Note

です is the polite version of だ, used when speaking to people you are not close with, superiors, or in formal situations. While だ represents a plain or casual tone, です shows respect to the listener. Remember that with nouns and な-adjectives, the past tense is formed by changing です to でした.

い-Adjective

Used after an い-adjective to make the sentence polite. In this context, です functions purely as a politeness marker rather than a true state-of-being copula.

い-Adjective + です / かったです

Appended directly to the present tense or the past tense form of an い-adjective to make a polite statement.

That watch is expensive. (polite)

Yesterday's movie was interesting. (polite)

Note

There is a major grammatical difference between です and だ when it comes to い-adjectives. The plain copula だ is ungrammatical after an い-adjective, but です can and must be attached to the end of an い-adjective to make a sentence polite. Note that for the past tense of an い-adjective, the adjective itself conjugates to the past form (〜かった) and is followed by the present tense です, not でした.

Words: 
です
Radicals of radicals