in English as with other languages, one word may have multiple meaning. It is important to understand the situation or context in which the word is used. for example: impotent - After the loss of his sight, Harry felt impotent. unable - Jesse was unable to work today because of the storm. (you would not use impotent in this sentence) incapable - It seems, the government was incapable of making right decisions. essentially these sentences are saying the same thing Finally; find the words with similar meanings and practice using them in as many ways as possible. hope this was helpful

Yes, as you advance in your English learning you must know the subtle differences between similar words. This is exactly what the English tests look for - your understanding of the differences. For your example, "I am (unable/incapable/impotent) to do it", the answer cannot be "incapable" because the sentence would need to change: "I am incapable of doing it". This is simply a collocation you need to learn. "Impotent" literally means "without power". Even though "I am impotent to..." is technically correct, it sounds a little absurd because it's so dramatic. You would use that word for very serious situations. So, "I am unable to do it" is your only sensible choice. As you learn and read more, you'll get to understand these subtleties.

You have to ask a few people the emotion behind the words. They can describe a scenario or a feeling that goes with the word and then you can get a feel for it. Keep in mind that words mean different things to different people. It all depends on the image they have in their mind.

Pay close attention to how merlyn totally misunderstood your question. You asked about definition and meaning. You used a hypothetical question, which had grammar problems, and merlyn focused on your hypothetical question with the grammar problem and completely did not address your question regarding meaning. Language is tricky. Have to pay attention to detail.