[sudarawee]

I am not a native speaker but please see below.

Conditional sentence is divided into 3 types.
1. Talk about something if it happens/ will happen in the present or future to express a probable condition. Or talk about the natural laws or habitual actions (general statement).
If he goes to London, he will meet his old friend.
If water is heated, it boils.
If students don't study they often fail.

2. Express doubt, express the improbable condition.
If i saw him, I should speak to him.
If I were you, I would cut down all those trees.
3. Talk about something in the past that is impossible to happen and it was opposite to the fact in the past.
If I had known of your arrival, I should have met you. (but I didn't know, so I didn't meet you)
If she had fallen into the river, she would have drowned. (but she didn't fall into the river, so she didn't drown)

Your sentence:
If I dropped my watch, it would break. It means you didn't drop your watch so it didn't break. It expresses the improbable condition
If I drop my watch, it will break. Shows probable condition. It may or may not break.
If I drop my watch, it breaks. It means this watch will surely be broken if you drop it.