Hello Alex. I believe that it is better to say " His eyesight is impaired because he spends a lot of time using the computer." Now if he already had a poor eyesight to begin with, you can say "Spending a lot of time using the computer has caused his eyesight to worsen." Impair means "to spoil something or make it weaker so that it is less effective." Worsen means "to become worse or make something become worse than it already was." I hope this helps. :)

This is where I do suggest checking a dictionary, as I suspect you are following what your translator has given you, without looking any deeper. http://www.thefreedictionary.com/worsen http://www.thefreedictionary.com/impair You'll see that "impair" is a transitive verb, so the question is "impair what?" So, it's not possible to write "His eyesight has impaired because..." The phrase is already missing information. Here are a few suggestions: "His eyesight has worsened because he spends a lot of time in front of the computer." "Spending a lot of time in front of the computer has worsened his eyesight." - This is technically correct, but a little awkward. "Worsen" can be transitive, but it's usually intransitive (this also means a naturally reflexive verb). You can use the gerund form "spending", and make the action the subject of the sentence. "Spending a lot of time in front of the computer has impaired his eyesight." - This sounds fine, but I think he is now partially blind.

Not interchangeable really. And you would say it "has worsened" not "it has worsen". As already mentioned it would depend on context. If his eyesight was already bad it can worsen or "get worse" as it implies that the situation was already bad. If you want to use impaired, you can say "his eyesight is impaired" or "he has impaired his eyesight" or that watching tv "has impaired his eyesight".

Implied = to say something without saying something. Worsen = something is deteriorating, (Worsen does not sound very good, normally you would say gotten worse, or become worse, or is worse because...) Impair = more temporary, generally as direct result of an action. but to answer your question I have changed your sentences slightly. His eyesight has become worse because he spends a lot of time in front of the computer. His eyesight was impaired because he spends a lot of time in front of the computer. or you could say, The time spent in front of the computer impaired his eyesight, or the time spent in front of the computer made his eyesight worse. I think the last option is the best, as eyesight deterioration is permanent and I think impair is a temporary worsening of something. PS Have you learnt about active and passive voice?