おかげで and せいで: gratitude and regret attached to causes
Compare おかげで crediting a cause for a good result with せいで blaming a cause for a bad one. Read negated result clauses, uncertain causes, and irony in context.
Check how the result is evaluated
友達が手伝ってくれたおかげで、早く終わりました。 means that the task finished early thanks to a friend's help. It evaluates the cause as contributing to a good outcome and can convey gratitude.
電車が遅れたせいで、約束の時間に間に合いませんでした。 means that the speaker missed the agreed time because the train was late. This evaluates the train's delay as the cause of an unwelcome result, not merely an earlier event.
Noun connections and negative result clauses
Nouns take の, as in 友達のおかげで and 雨のせいで. A verb can connect as 手伝ってくれたおかげで, and a na-adjective construction can use 静かだったおかげで, with the connecting form and tense chosen to fit the meaning.
早く出たおかげで、遅刻しませんでした。 means that leaving early prevented being late. Although the result verb is negative, avoiding lateness is a good outcome, so it fits naturally with おかげで.
Judge certainty and irony separately
寝不足のせいか、今日は集中できません。 means that the speaker cannot concentrate today, perhaps because of insufficient sleep. The か in せいか makes the cause tentative, unlike a definite report of causation.
君のおかげで、ひどい目にあったよ。 can sarcastically blame someone for a bad experience while literally saying thanks to you. Learn the basic meaning first, then identify irony from an actual mismatch between the result and tone.
Quick check: Does 傘を借りたおかげで、ぬれずに済みました。 describe an unwelcome outcome?
Show the answer and why
No. Borrowing an umbrella allowed the speaker to avoid getting wet, which is a good outcome. Do not choose せいで simply because ぬれず is negative.