Can able

Correct vs. Incorrect:

I can do it  -  Correct.
I am able to do it  -  Correct.
I will be able to do it  -  Correct.
I can able to do it  -  Incorrect but many people use it.

Why is the last one incorrect? The answer will follow...

Why is 'I can able to do it' incorrect?

Answer: The word 'can' is an auxiliary verb which should be followed by the main verb only. But the word 'able' is an adjective. Hence, it is incorrect. By the way, what are main verb and auxiliary verb? See below:

A verb in a sentence has two parts, namely auxiliary verb and main verb wherein auxiliary verb is optional. The form of the verb is auxiliary verb + main verb (form). It is illustrated below:

I did it.  - 'did' is main verb without auxiliary verb - the tense of the verb is past.

I am doing it.  - 'am' is auxiliary and 'doing' is main verb - the tense of the verb is present continuous.

I can do it.  - 'can' is auxiliary and 'do' is main verb - the verb is not in any specific tense but such usage is called modal because it is not specifically related to time but conveys just a possibility / capability.

See you ...