～せいで— Because Of (And I Blame It)

• 電車が遅れたせいで、会議に遅刻した。

  Densha ga okureta sei de, kaigi ni chikoku shita.

  Because the train was late, I was late for the meeting.

  Verb past 遅れた + せいで blames the delayed train for the bad result.

• 君のせいで計画が台無しだ。

  Kimi no sei de keikaku ga dainashi da.

  Because of you, the plan is ruined.

  Noun 君 + の + せいで places blame directly on the listener.

• 暑いせいで、よく眠れなかった。

  Atsui sei de, yoku nemurenakatta.

  Because it was hot, I couldn't sleep well.

  い-adj 暑い + せいで attributes the poor sleep to the heat.

• ・Always frames the result as negative; carries blame or complaint.
• ・せいだ can end a sentence: 失敗は準備不足のせいだ.
• ・For a positive result, switch to おかげで.

Commonly confused with

The JLPT rarely asks whether you recognise a pattern — it asks which of several similar patterns fits. These are the ones ～せいで is tested against.

• ～あまり(に) — So Much That It Overwhelms
• ～おかげで — Thanks To
• ～せいか — Perhaps Because Of
• ～からといって — Just Because
• ～ことから — Judging From The Fact
• 次第だ／次第で — It All Depends
• だけあって — Living Up to It
• ～につれて — As One Thing Changes, So Does Another