Copular verb です
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 "