"It's all I ever wanted."
Translation:זה כל מה שאי פעם רציתי.
I'm wondering why we use "כל" rather than "הכל", which sounds more correct to me. I would have expected to use "הכל" but not use "מה".
"זה הכל שאי פעם רציתי".
Is that a valid alternative?
The word כל is a quantifier and שרציתי is a qualifier. You need a noun in between. When referring to unspecified things the noun is מה. When referring to unspecified people the noun is מי.
In English, "everything" can be qualified. In Hebrew, הכל is never qualified.
Would this same sentence using זאת be incorrect? What if the "thing wanted" is a feminine noun?
No, it would still be זה. I suppose the reason is that זה agrees with the noun that follows it rather than the noun preceding it. So in this case it agrees with כל מה שאי פעם רציתי. You could say היא כל מה שאי פעם רציתי.
Following your reasoning, in your second example היא, being feminine, agrees with כל מה שאי פעם רציתי, which in that case would be feminine. What am I missing?
Oh right, I didn't write: the pronoun copula acts differently, it agrees with the first noun. So עורב הוא ציפור, but עורב זו ציפור. And if the first noun is a pronoun, we don't repeat the pronoun, we say היא/הוא כל מה שרציתי depending on the grammatical gender of the subject; but if the subject is "this", we say זה כל מה שרציתי regardless of the grammatical gender of the subject.