What is the past tense of the word 'now'?

There is no past tense of the word "now," because past tense is a verb form that expresses action that has already occurred.

"Now" is an adverb, not a verb. Adverbs modify a noun, verb or phrase.

For example: In the question "What time is it now?" "now" modifies the noun "time."

The antonym (opposite) of "now" is "then."