Why use で in「私でよければ入部させてください」?

Question

What's the difference between these sentences:

• 私でよければ入部させてください

• 私はよければ入部させてください

• 私をよければ入部させてください

I was watching the first episode of K-On! anime and I saw this line

私でよければ入部させてください.

The English subs translated to

If it's okay with you I'd like to join the club.

The part that confused me was the 私で. I got as far as understanding that the person that is doing the "allowing" is the person being spoken to, the club leader meaning the verb is being applied to an unseen subject but I still don't understand why mark the person being allowed with で. It makes no sense to me.

Answer 1

～でいい (or ～で大丈夫, etc) is an expression that means "～ is acceptable (if not ideal)". I think this is something you can learn by rote. See:

• What is the difference between それでいい and それがいい here?
• Maggie Sensei: Noun + で(も）いい　VS +がいい

So the literal translation is:

私でよければ入部させてください。
If I'm acceptable (to you), let me join the club.

And the implication of the first half is "although I may not be perfect/ideal". Something like this is said when the speaker is trying to be humble or when she is not very confident.

私はよければ is ungrammatical simply because は is not allowed in subordinate clauses.

私をよければ入部させてください means "Let me, if it's okay, join the club." The word order is not very natural but it still can be parsed as a variation of よければ私を入部させてください ("If it's okay, let me join the club"). This no longer has the original implication of "although I may not be perfect"; the speaker understands she is not the decision maker, but she is thinking she will serve as a suitable member.