Plain form (na/no) + せいで/せいか → B
Plain form (na/no) + せいだ
Indicates that A causes a negative outcome B

Verb plain form/i-adjective plain form + せいで/せいだ/せいか
Na-adjective stem + na
Noun + na/no
Usage: Blames A for causing negative outcome B, with strong tone of regret or complaint

Key Points

1. ～せいか indicates uncertain cause
2. ～を～せいにする means "to blame something on" (e.g. 自分の失敗を人のせいにする - blame one's failure on others)

Example Sentences

• 友達に話しかけられたせいで、授業に間に合いませんでした
  I was late for class because my friend talked to me.
• 背が高いせいで、頭をドアにぶつけてしまいました
  Because I'm tall, I hit my head on the door.
• 仕事が大変なせいで、恋人に会えません
  Because work is demanding, I can't see my partner.
• 怪我のせいで、試合に出られません
  Due to my injury, I can't participate in the match.
• 年をとったせいか、忘れっぽくなりました
  Perhaps because I'm getting older, I've become forgetful.
• 机の上が汚いせいで、必要な資料が見つかりません
  Because my desk is messy, I can't find the documents I need.
• 熱があるせいで、何を食べても美味しくないです
  Because I have a fever, nothing tastes good.
• 後輩の失敗のせいで、私まで叱られました
  Because of my junior's mistake, I got scolded too.
• 今日は雨のせいか、いつもよりお客様が少ないと思います
  Perhaps becuase it's raining to day, there seem to be fewer customers than usual.

Usage Notes

This grammar pattern is used when:

• The speaker wants to attribute responsibility for a negative outcome
• The cause is known (せいで) or uncertain (せいか)
• The result is clearly negative (being late, getting hurt, etc.)

Sentence Construction

// Verb example
話しかけられる (passive plain form) + せいで → 話しかけられたせいで

// I-adjective example
高い (plain form) + せいで → 高いせいで

// Na-adjective example
大変だ → 大変な + せいで → 大変なせいで

// Noun example
怪我だ → 怪我の + せいで → 怪我のせいで