I like mechanical watches. I like mechanical watches.

I like mechanical watch. I like mechanical watch.

I prefer mechanical watches.

I like the mechanical watch. I like the mechanical watch.

You need a little bit more information to specify which watch you are referring to. You could use 'a watch', 'the watch', 'my watch', etc.

First, I like the appearance. First, I like the appearance.

First, I like appearance. First, I like appearance.

First, I like the appearance.

First, I like the appearance. First, I like the appearance.

I tried to make thisone when I was child. I tried to make one when I was child.

I tried to make thisone when I was child. I tried to make one when I was child.

'this' can work here in theory, but only if you tried to make the exact same watch when you were a child

But I caouldn't make this, because this is too complicated. But I couldn't make this, because this is too complicated.

can't = can not which is present tense couldn't = could not which is past tense

But I caouldn't make ithis, because this iit was too complicated. But I couldn't make it, because it was too complicated.

Same as above. 'This' has to refer to one specific object, and it is usually used as a more general reference

Maybe I can make ithis now. Maybe I can make it now.

this is more specific, so if you had a watch kit in front of you, you could refer to it as this (one) It is more general, maybe you could make a mechanical watch in the future

Maybe I can make thisone now. Maybe I can make one now.

I need to more knowledglearn more about mechanical watches. I need to learn more about mechanical watches.

I need to more knowledge about mechanical watches. I need more knowledge about mechanical watches.

If I have a lot of free time, I will try it. If I have a lot of free time, I will try it.

If I have a lot of free time, I will try it. If I have a lot of free time, I will try it.