In the book that I am learning, there was a sentence:
I had my watch mended.
I don't know what that means. Does it mean the following?
I commanded that my watch be fixed.
"To have something done" can carry a lot of sub-text and hidden messages in some cases, when it is compared to alternative wording. However, In the particular case of this question, it seems to me that it is not so "loaded".
A likely meaning of this sentence is "I gave my watch to someone at the shop because it was broken; then I got it back and it was mended."
The passive phrasing of this sentence suggests that the speaker might not know (or care about) who actually did the mending. This is a typical event in modern times when we mostly interact with front-desk representatives and don't know the back-office workers... So the passive phrasing is more suitable than the alternative active "someone mended my watch for me".
I had my watch mended
means that you got your watch repaired. Here, 'had' means 'got it done' and 'mended' means 'fixed/ repaired'.
In short, it simply means:
I got my watch repaired.
I got my watch fixed.