The particle で has a range of functions in Japanese. Fundamentally, it marks an element that is necessary to carry out an action or achieve a result. One of its key roles is to indicate the 'thing' used, which translates to 'by' or 'with' in English.

• 自転車(じてんしゃ)で会社(かいしゃ)に行(い)く。
  I go to the office by bicycle. (Required to reach a goal)
• 飛行機(ひこうき)で旅行(りょこう)する。
  I travel by airplane. (Required to reach a goal)

In these examples, the specified item is crucial for completing the action. Even though other methods might exist, で points out the specific one that was selected.

Additionally, で is used when an action is performed by a group. This can be tricky for learners at first, as と also means 'with' when referring to people. However, when で is used in this way, it carries the same nuance of 'means' or 'instrument', suggesting that the entire group acts as a single unit to accomplish the task.

• 家族(かぞく)で夕食(ゆうしょく)を食(た)べる。
  The family eats dinner together. (Being with family is what allows/gives purpose to the action)
• 友達(ともだち)で海(うみ)へ行(い)きたい。
  I want to go to the beach with my friends. (Being with friends is what allows/gives purpose to the action)

The key difference between using と and で for 'with' lies in the group's purpose. で is chosen when everyone in the group shares a common goal or destination. In contrast, と is used when the focus is on the interaction between individuals, rather than a collective, external objective.

• クラスみんなで旅行(りょこう)に行(い)った。
  The whole class went on a trip together. (Being with everyone is what allows/gives purpose to the action)
• 先生(せんせい)と話(はな)した。
  I talked with my teacher. (Being together is the objective, talking is just what is happening while being together)