"one" vs "it", "they" vs "ones"

Question

I don't know how to to choose correct one.

I know I can say:

I like fishing and I'm good at finding "it" in the river.

I brought some apples because you like "them".

I brought this sort of apples because you like sour "ones".

I've lost my umbrella, and have to buy new "one".

Then, how can I say in the situations below:

1. Yes, I'm good at finding birds. I can catch one later if you want "one/it" for a pet.

2. There were no birds in the first and second tree, but I found "two of them/two" in the third tree.

Answer 1

The first example doesn't work. The singular "it" would seem to refer to "fishing"... but you don't find "fishing" in a river. You might find "fish", but that is a plural noun (many fish).

In the birds example, I'd say "one", though both could be correct. I feel that "If you want it..." seems to refer back to a specific bird, but since you haven't caught the bird yet, you don't know yet which specific bird you will catch. That is "it" means "the bird", but no bird has yet been determined.

In the second sentence both form are correct. I'd use "I found two in the third tree", because it is shorter.

Answer 2

Yes, I'm good at finding birds. I can catch one later if you want it for a pet.

Neither "one" nor "it" is wrong here. However, since you used "one" only a few words before, using a different word sounds better. The differences in meaning between the two sentences are very small, not enough to cause any misunderstanding. "One" tends to be a more general "any one will do" meaning while "it" is more targeted. It's like the difference between saying "a bird" and "that bird".

There were no birds in the first and second tree, but I found two of them/two in the third tree.

Again neither is wrong, and both sound equally natural. Simply using "two" is better in general since adding "of them" isn't necessary; we know that "two" refers to the birds.