How you do something: で
で can mark the means of an action: transport, a tool, or a language.
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Use で after the means or tool: 電車 で 行きます means “I go by train,” and 箸 で 食べます means “I eat with chopsticks.”
English changes its preposition: by train, with chopsticks, in Japanese. These uses can all take で. Do not confuse “with a tool” with “with a companion”: a companion uses と.
Transport
バス で 行きます
I go by bus.
Tool
箸 で 食べます
I eat with chopsticks.
Language
日本語 で 話します
I speak in Japanese.
Examples
けんさん は 自転車 で 学校 に 行きます。
Ken goes to school by bicycle.
鉛筆 で 書きます。
I write with a pencil.
日本語 で 話します。
I speak in Japanese.
何 で 帰ります か。
How are you getting home?