Difference between "had/has/have"

[tsiddik]

They are able to do this.

They have able to do this.

They were able to do this.

They had able to do this.

please differ the meaning of these above sentences explicitly.

[5jj]

Welcome to the forum, tsiddik.

They are able to do this. Present tense.
They have able to do this. Not a possible construction.
They were able to do this. Past tense.
They had able to do this. Not a possible construction.

Did you mean to write 'They have been able to do this' (present perfect) and 'They had been able to do this' (past perfect)?

[tsiddik]

Eventually,they have been able to show their best credentials.

Eventually,they had been able to show their best credentials.

Just differ the meaning between them?

[5jj]

No. I have already said that 'not able' is not a possible construction.

(later) I see that you have now added 'have/had' to your previous post.

[tsiddik]

please differ the meaning of them,this is my priority.

Eventually,they have been able to show their best credentials.
Eventually,they had been able to show their best credentials.

[Mohammadhelmi]

[QUOTE=tsiddik;895115]They are able to do this.

They have able to do this. this structure is unacceptable

They were able to do this. specific ability in the past at a particular situation

They are able to do this. general ability at present

They have able to do this. this structure is unacceptable

[alenjones23]

They are able to do this - Sentence with present tense

They have able to do this - Error in sentence formation

They were able to do this - Sentence with past tense

They had able to do this - Error in sentence formation

[Barb_D]

please differ the meaning of them,this is my priority.

Eventually,they have been able to show their best credentials.
Eventually,they had been able to show their best credentials.

I don't know why two new posts have come since your last question which only repeated what has already stated, but neither of these work.

Eventually, (at some point in the past, although it took some time to accomplish it) they were able to show their best credentials.

Eventually, (at some point in the future, although it will take you some time to accomplish it) they will be able to show their best credentials.

"Eventually" does not work with the present or past perfect.

I should also say that the expression "able to show their best credentials" doesn't mean anything to me.