I dont understand this piece of grammar

I don't get why this sentence has "eigo wo hanaseru" in it instead of  "eigo ga hanaseru".
I thought "ga" was preferred when you use verb forms that mean "i can talk/etc..."
Is this because this phrase works as a modifier to "hito"?

英語を話せる人はいますか？
Eigo o hanaseru hito wa imasu ka?

Hi Nick!

I doesn't makes sense merely from what you said. I kinda need to know the general rule for when to swap ga for wo, don't I? But someone explained a similar situation like this to me: If you look at the whole sentence you really have:

(eigo wo hanaseru hito) ga/ha imasu ka?

Where the whole parenthesis part is the subject, and ga marks that. So ga cannot be used inside that part also or you will have two subjects. So here wo is used instead of ga, even though "ga" is usually used with potential verbs like hanaseru and dekiru.

Would you say this is that kind of situation?