Imagine that it's your father's birthday. Look at the photographs and think which of these things you would buy him.
I would buy him a watch.
Nice job, but maybe:
I would buy him the my father a nice watch.
I would buy him a nice watch.
Everyone has pointed out the errors, but they didn't explain why. You need to use "a nice watch" because it means "any watch" not a specific one. "the"= specific, "a" and "an" = general. It is also the first time you mention the watch. On the second mention, then you can use "the" because the watch has become a specific one, the one that you bought for your father.
For example, "I wanted to buy my father a watch for his birthday. When I went to the department store, I looked for the watch he said he wanted. Then I paid for the watch at the cash register. When I got home, I wrapped up the watch beautifully.
You see? The first time you talk about it..... use "a/an". After that, use "the".
Notice the green words. I used the for "the department store" and "the cash register". Although this is the first mention, using "the" indicates "the department story I usually go to". They listener understands which one. Using "the cash register" means the specific cash register at that specific store. Maybe there is only one, but the listener understands which one you mean.
Hope this helps.
Thank you for your help!