What's the difference between "only one" and "single"?
Let's use this sentence as example:

"You are the only one who is able to do that."

Could I use "single" in replace of "only one"?

Help me about distinguishing only one and single.

When do I use "single" instead of "only one"?

Thank you very much.

Question

What's the difference between "only one" and "single"?
Let's use this sentence as example:

"You are the only one who is able to do that."

Could I use "single" in replace of "only one"?

Help me about distinguishing only one and single.

When do I use "single" instead of "only one"?

Thank you very much.

Answer

They are grammatically different. '[The] only one' is a noun phrase, while 'single' is an adjective. In your example, 'the only one' means 'the only person'. An adjective, such as 'single', can never replace a noun.

The word 'single' has to qualify a noun, for example, 'the single reason'.

Answer

No, because you want a noun or pronoun there, which "only one" is, and "single", being an adjective, isn't.  (Well, there is "single" as a noun meaning an unmarried person, but that doesn't apply here.)  You could however say "you are the single person ...".