A1
で — where it happens, and how
How で marks the setting of an action and the means of doing it in Japanese, and the に versus で distinction that decides which to use.
で marks where an action takes place, or what it is done with.
レストランで食べます。
Resutoran de tabemasu.
We’ll eat at a restaurant.
うちで映画を見ます。
Uchi de eiga o mimasu.
I watch films at home.
Means
The instrument, the material, the method, the language.
バスで行きます。
Basu de ikimasu.
I’ll go by bus.
日本語で話しましょう。
Nihongo de hanashimashō.
Let’s speak in Japanese.
はしで食べます。
Hashi de tabemasu.
I eat with chopsticks.
で against に
The one to get right. で is where something happens; に is where something is. The place can be identical and the particle still differs.
図書館で本があります。
図書館に本があります。
あります states existence rather than an action, so the library is a location and not a setting. Swap the verb for one that does something — 図書館で本を読みます — and で is correct again.
A useful test: if the verb is あります or います, it is に. If something is being done, it is で.
で also closes a cause
A minor use worth recognising early, since it turns up constantly in the news and in excuses.
風邪で休みました。
Kaze de yasumimashita.
I took the day off with a cold.