Copular verb です

Copular verb です(desu)

Copular verb desu change its form based on the formality of speech (plain or polite),

polarity (affirmative or negative), and tense (non-past or past).

Plain form are used in a plain/informal speech context.

Polite form are used in a neutral/polite speech context and they end in ます (masu), です(desu),

and their variations.

Past tense refers to actions that took place in the past or that have completed.

The non-past tense refers to future actions, habitual actions, and actions that have not started yet.

Desu can directly follow a noun or an adjective to show equality.    

Desu is used in making the statement  "A is B" or "As for A, it is B "