Why "I had been"? The speaker *is* an only child (now), so shouldn't it be "I were"?

"had been" is OK here.
Past perfect tense is used for wishes and hypotheses in the past, e.g. "I wish I had worked harder when I was at school."

see: http://learnenglish.britishcoun...and-hypotheses

Thanks for your comment, Pat.

I know this is a correct English sentence. What I wanted to say is that I'm not sure if it really matches the Japanese.

"I wish I were an only child." is also a correct sentence, isn't it? Could you explain how it's different from the version with "had been"?

Tommy_san, just for my understanding
Did you want to say >> The speaker *is* an only child (now)<<?
In my understanding of the English sentence (of both versions - 'were' and 'had been') the speaker is not an only child, he or she just wishes to be one.

Even though the original Japanese sentence ends with "ta" (た), which is often used to indicate the past tense, the "ta" here merely means, I believe, that it's about an unreal hypothesis. I'm not sure whether Japanese has a grammatical distinction between "I were" and "I had been".

I overheard the sentence in a train. A small girl told it to her friend and then complained how she was unhappy because of her brother.

@brauchinet

Oh, yes, you're right. I wrote it wrong. I just wanted to say that it's a wish about the present.
I think the Japanese means that the speaker would be happier now if s/he were an only child.

I see. This is in the present, not the past. I don't know if that makes my translation wrong or not. I'll unlink it anyway.