で (de) — Meaning & Examples

で has two main uses that are easy to understand once you see the pattern:

1. Where an action takes place:
としょかんで べんきょうします。 — I study at the library.
レストランで たべます。 — I eat at the restaurant.
かいぎしつで はなします。 — I talk in the meeting room.

に vs で for location:
This is a common point of confusion:
に = where something EXISTS (います、あります)
で = where an action HAPPENS

ねこは へやに います。 (The cat IS in the room — existence → に)
へやで ねます。 (I sleep in the room — action → で)

2. Means / method / material:
バスで いきます。 — I go by bus.
にほんごで はなします。 — I speak in Japanese.
はしで たべます。 — I eat with chopsticks.
ひとりで やりました。 — I did it alone.
えんぴつで かきます。 — I write with a pencil.

The "by means of" で is very commonly used — anytime you want to say how you did something, で is usually the answer.