How many ways to say 'when' in Japanese?

There are 3 ways: 時に toki ni, 頃 koro and 時代 jidai.

時に is used to describe a specific time or event that occurred in the past.

頃 is used to describe a broader time than 時に, which is for specific times.

時代 means "era" or "period" and used to describe a specific historical period or era.

What is the grammar for nouns with 時に?

For nouns, you add の in between the noun and 時に.

Our Japanese Grammar Bank has some fantastic free resources for you to check out.

Grammar Point	Pattern	Example
Expressing when in Japanese	Verb / Adjective / Noun (+ の) + 時に	私は暇な時に映画を見ます。
Expressing certainty in Japanese	Verb / Nouns / Adjectives + だろう	この店の料理は美味しいだろう。
Expressing only in Japanese	Verb / Adjective / Noun + だけ	一回だけ行きました
Expressing in order to in Japanese	Purpose/Goal/Person Benefiting + のために + Verb	彼は新しい仕事に備えるのために勉強しています。
Expressing as soon as in Japanese	Verb + が早いか	彼は試合が始まるが早いか、ホームランを打った。