[susan53]

These words are horribly complex, so don't worry if they confuse you. Looking at the grammar books always makes me think they've been forced into grammatical categories that they don't really fit. But anyway ...

Everything is usually classed as a pronoun - to be more precise a quantifying pronoun. Which puzzles me, as if by chance the language had decided to write it as two words - every thing - it would be a quantifier + noun. But there you go ...

Anyway, let's see it as a pronoun. That means it can fit into either subject or object position :
Everything is OK. / I didn't finish everything
(or, as in your example : He couldn't provide her with everything she needed.)

It's a pronoun because it's substituting for other understood items :

I had to clean the kitchen, make the beds, hoover the carpets and do the washing. but I only had half an hour and I didn't finish everything.

Everything substitutes for clean the kitchen, make the beds, hoover the carpets and do the washing

All on the other hand is a quantifier which can have various grammatical functions :

- Adverb : used to modify a preposition (all over, all round, all through etc) or adjective (all dirty, all right) etc : He painted all through the night / He got all dirty.
- Determiner : All the people cheered./ I don't like all his books
- Pronoun, as in your example : He couldn't provide her with all she needed

Even in the last example though, it makes more sense to me to think of all as really being a determiner with the noun phrase elided (or omitted) - ie :He couldn't provide her with all (the things) she needed.