～せいで／せいか— Blame It On…

• 台風のせいで、電車が止まってしまった。

  Taifuu no sei de, densha ga tomatte shimatta.

  Because of the typhoon, the trains stopped.

  台風のせいで blames the typhoon for the negative result

• 寝不足のせいか、頭が痛い。

  Nebusoku no sei ka, atama ga itai.

  Maybe because I didn't sleep enough, I have a headache.

  せいか adds uncertainty — lack of sleep is a guessed cause

• 彼が遅刻したせいで、会議が始められなかった。

  Kare ga chikoku shita sei de, kaigi ga hajimerarenakatta.

  Because he was late, we couldn't start the meeting.

  遅刻したせいで assigns blame for the failure to start

• ・せいで assigns blame for a negative result; せいか expresses a tentative guess.
• ・Opposite of おかげで (thanks to), which is for positive results.
• ・Noun connects with の (病気のせいで); verbs/adjectives use plain form.

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／せいか is tested against.

• に伴って — Hand in Hand With Change
• ～以上(は) — Now That, Given That
• ～からこそ — Precisely Because
• ～からには — Now That, Since (With Resolve)
• ～ことだから — Knowing Them, Surely…
• ～だけに — All the More Because
• ～だけのことはある — Living Up to It
• ～もの／もん — Because, You See