〜せいで／せいか
Because of… (cause of a bad outcome)machine translation
FormationVerb/い-adjective plain form + せいで; な-adjective + なせいで; noun + のせいで
〜せいで gives the cause of a BAD outcome, with a tone of blame or reproach: 事故のせいで遅れた (I was late because of the accident). Unlike neutral から・ので, the moment you use せいで, the listener understands you are annoyed or laying responsibility on that cause.
Connection: noun + のせいで, な-adjective + なせいで, and verbs and い-adjectives go directly with the plain form. The variant 〜せいか means "perhaps because…" — the speaker isn't sure about the cause: 年のせいか、疲れやすい (perhaps because of my age, I tire easily). There is also 〜のせいにする (to blame … for something): 人のせいにしないでください (don't blame others).
An opposing pair to remember: おかげで is used for GOOD outcomes with gratitude, せいで for bad outcomes with blame. Mixing up the direction completely changes the sentence's attitude, so always ask yourself whether the outcome in the sentence is something you're glad about or upset about.
Examples
事故のせいで、電車が二時間も遅れました。
Because of the accident, the train was a whole two hours late.
夜遅くまでゲームをしたせいで、今朝は起きられませんでした。
Because I played video games until late at night, I couldn't get up this morning.
年のせいか、最近疲れやすくなりました。
Perhaps because of my age, I've been tiring easily lately.