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 ...".

لا تفوّت فرصة تعلّم لغة جديدة وأنت مرتاح في منزلك. تصفّح مجموعتنا المختارة من مدرّسي اللغات ذوي الخبرة وسجّل في درسك الأول الآن!