この時計を見てください。Look at the watch. 特定な物と話しているなら、"the”　を使います。どっちでも物を話しているなら、"a"　を使います。When ever it is a specific item, use "the." If you mean watches in general, then use "a." Please look at the watch. (if it is a specific watch, such as a silver apple watch) Please look at a watch. (if it is not a specific watch)。 If I want to ask my friend about the time, then I would say "What time is it" instead of "look at the watch" 友達に今の時間を聞きたいなら、「この時計を見なさい」の代わりに　「今何時？」といいます。