Is the sentence below correct?
I want to not only be able to...
Or is it: I want not only to be able to...
2nd one is correct
Right. Don't split the infinitive -- to be able.
or just "to be"
The sentence "I want to not only be able to..." is grammatically correct. It is structured as follows: subject ("I") + verb ("want") + infinitive verb phrase ("to be") + coordinating conjunction ("not only") + auxiliary verb ("be") + infinitive verb phrase ("able to"). This construction is commonly used to emphasize that the desired action is not the only one, but rather just one of several things.
Alternatively, you can also rephrase the sentence as "I want not only to be able to..." which also maintains correctness. The only difference is in the placement of the "not only" phrase. In this construction, the "not only" phrase is placed before the infinitive verb phrase instead of before the auxiliary verb. Both versions convey the same meaning, so you can choose the one that sounds better to you stylistically.