For example, if I have difficulty listening, I might ask one of the following questions:
- How to improve that?
- How can I improve on that?
- How do I improve on that?
Which of these questions is more correct to say in this situation?
I'd also be glad to know if any are incorrect.
I think the natural phrasing would be, "How can I improve that?" or "How do I improve that?", or depending on context you might use a generic "you" and say, "How can you improve that?", etc.
Your versions 2 and 3 are grammatically correct. Version 1 is not correct because it has no subject. Who is doing the improving?
The first question, 'How to improve that?' is a sentence fragment; it's incomplete. Also, it's asking the wrong question. "How to improve that ... I have difficulty in listening" ? How to improve ... the difficulty? You want to know how to make it more difficult to listen? I would think what you want is to improve your listening, not the difficulty. But the word that is ambiguous in this phrase.
When you add the subject "I" and the preposition "on", as in "How can I improve on that?" You slightly improve the ambiguity. "How can I improve on ... I have difficulty in listening."
To be clear, you'd make a full sentence of something like "How can I improve my listening skills?" Or something similar, but if you need to use 'that', the latter two examples are fine.
