で

cause

Marks the cause of a result: "because of X" or "due to X". The result is usually something nobody chose.

Readings

How to form it

Put で after a noun for the cause: 風邪(かぜ)で休(やす)んだ (I was off because of a cold), 地震(じしん)で電車(でんしゃ)が止(と)まった (the trains stopped because of an earthquake).
で only follows a noun. For a whole clause as the reason, use から or ので.
The second half reports what happened, so this で does not come before a request. Say 雨(あめ)だから、傘(かさ)を持(も)って行(い)ってください.

風邪(かぜ)で学校(がっこう)を休(やす)みました。

I missed school because of a cold.

事故(じこ)で道(みち)が混(こ)んでいます。

The road is busy because of an accident.

〜ためにbecause of
"Because of" in formal speech and writing: 事故(じこ)のために.
〜せいで / 〜おかげでbecause of / thanks to
"Because of" with blame (せいで) or gratitude (おかげで).

でmeans / tool / material
The same で after a tool or vehicle means "by" or "with".
でlocation of action
The same で after a place marks where an action happens.