Japanese verbs change their forms based on the formality of speech (plain or polite), polarity (affirmative or negative), and tense (non-past or past).
Plain forms are used in a plain/informal speech context.
Polite forms 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.