So I was studying this book called 日本語生中継, and there was this question where you're supposed to choose the inappropriate word among others. And I got stuck at this sentence:
私は、パソコンが(得意だ.わかる.上手だ.)
The book says the correct, or rather, incorrect answer, was 上手だ. But I don't seem to understand why, as all three seem correct to me.
I am a native Japanese speaker so I think I have been properly differentiating these two words unconsciously, but I couldn't really articulate what makes them different, so I looked up. Here I briefly summarize the article cited below.
The difference between 得意 and 上手 can be illuminated most when you translate into Japanese the following sentence: "This novel is well written."
この小説は上手に書けている。
この小説は得意に書けている。
While the former is perfectly natural, the latter is simply wrong. This is contrasting with the sentence from the OP. The question is, why the difference?
It appears that 得意 is used to describe the (good) skills to do/produce something, while 上手 is used to describe the results of the (good) skills put to use. In the example above, the former sounds natural since the novel is a product of the writer's skill. On the other hand, the latter does not make sense since the novel itself is not a skill; it's a product of a good skill. Perhaps, a similar reasoning leads a somewhat rigid Japanese language teacher to think that 私はパソコンが上手だ to be technically wrong, since a personal computer is not a product of skill itself.
However, I find it's totally natural to hear that sentence myself. After all, many Americans say "I don't have no money." While it's grammatically incorrect, it sounds natural and accepted to a certain group of people, right? :P
Seriously, in a way, 私はパソコンが上手だ is perfectly natural since you could argue that here it's totally expected that パソコン means パソコン（を使うこと）, and a variety of skills can lead to using a computer well, such as drawing on Photoshop, programming games, making music, using Excel, etc. So the reason why we feel the usage is natural could be because our notion of using computer has changed. Computer can do a lot of things, and we attach a lot of different meanings to (using) computer. Just my feeble thought...
The other answer to this question is wrong and misleading concerning the textbook where this problem is originating from. Don't trust it, as I did. I am at a well-known Japanese university doing Japanese language courses where we also use the textbook this question is from. I was doing my homework and got stuck at this very question, just as you. I read the incorrect answer and told the native Japanese teacher who looked at me puzzled. She pointed out that I, or rather the answer provided by Taro Sato is missing a crucial element.
The actual answer is: You can't use 上手 with 私. It is really that simple. This may not be a grammatical error, but it sounds extremely braggy and Japanese people just don't use it. It is important to point out that the other answer is wrong because it's perfectly fine to use 上手 in the same situation talking about someone else. THIS HAS NOTHING TO DO WITH BEING A SKILL OR A VERB OR NOT. For example 田中さんはパソコンが上手らしい。That is completely acceptable, according to my native university level Japanese teacher who has been teaching for several decades. Similar rules apply for 下手. Use it for other people, but use 苦手 when talking about yourself, although this is more common than 上手 because stating that you can't do something is not a brag. I hope this helps people in the future not to be embarrassed in class.
To clarify, I am not saying that the connotational difference pointed out in the other answer does not exit, it very well might be the case. I just wanted to provide an answer based on the problem in the textbook, which is quite clearly hinting at the difference between referring to oneself and others, at least according to my teacher.
