I was wondering if both of them means the same in the following sentences, that is 'all things I can'.
I'll do all I can to help.
I'll do everything I can to help.
If so, can I replay 'everything' with 'all' in the sentence 'We have completely different opinions. I disagree with everything she says.'
Best Answer
all and everything have the same meaning. everything is only a pronoun, but all can be a determiner, pre-determiner, pronoun and adjective.
For pronoun usages, either is usually acceptable, though all is rather more formal, for example:
The relative pronoun that is often omitted in informal speech and writing: your first two sentences could be written as
Because all is more formal, that is less likely to be omitted after all than after everything, especially in written English. This NGram graph shows the effect quite nicely.
Looking at your third sentence, it is perfectly possible to use all in place of everything but, because all is more formal, you would be less likely to omit the relative pronoun in the all version, especially in writing.