Grammar JLPT Level N5 - Lesson 9

で is a particle that has several different uses in Japanese. At its core, it is always used to highlight something that is 'required' to perform some sort of action/reach some kind of goal. One of these uses relates to 'things'. This comes across as 'by' or 'with' in English.

• 車(くるま)で空港(くうこう)へ行(い)く。
  I go to the airport by car. (Required to reach a goal)
• 電車(でんしゃ)で行(い)く。
  I go by train. (Required to reach a goal)

In these sentences, the item is considered essential for the action to be performed. While other items may be used, で simply highlights the one that was chosen.
で is also used when groups of people perform actions together, this is something that many people learning Japanese have trouble with initially (because と may also be used as 'with' for people). However, the use of で is actually the same as this by/with, meaning that the speaker considers the group as the tool used for performing the action.

• みんなでレストランに行(い)く。
  I go to the restaurant with everyone. (Being with everyone is what allows/gives purpose to the action)
• 来年(らいねん)は皆(みんな)でハワイに行(い)く。
  I will go to Hawaii with everyone next year. (Being with everyone is what allows/gives purpose to the action)

The difference between と and で as 'with', is that で is used when everyone in the group has the same objective/end goal. と is used when each person is performing the action toward each other, not toward an external objective.

• 家族(かぞく)全員(ぜんいん)で沖縄(おきなわ)へ行(い)く。
  I will go to Okinawa with everyone in my family. (Being with everyone is what allows/gives purpose to the action)
• お母(かあ)さんとご飯(はん)を食(た)べる。
  I eat with my mom. (Being together is the objective, eating is just what is happening while being together)