Learn Spanish (Latin America): Mi reloj nuevo deja de funcionar a veces.

The phrase dejar de followed by an infinitive verb (like funcionar) is a set expression that means 'to stop doing something'. The de is a required part of this phrase. Without it, dejar usually means 'to leave' (an object somewhere) or 'to let/allow'.

The English translation says 'working', so why isn't it funcionando?

In Spanish, when a verb comes immediately after a preposition like de, it must always be in the infinitive form (funcionar). Even though English uses the '-ing' form ('stops working'), Spanish pairs dejar de with the infinitive.

Why does the sentence use funcionar for 'working' instead of trabajar?

In Spanish, trabajar is used for physical or mental labor done by people. When talking about a machine, device, or system (like a watch) doing what it is supposed to do, you must use funcionar (literally, 'to function').

Why is it reloj nuevo instead of nuevo reloj?

In Spanish, descriptive adjectives usually go after the noun, so 'new watch' becomes reloj nuevo. If you put nuevo before the noun (mi nuevo reloj), it means 'newly acquired' or 'a new watch to me' (even if it is a used watch). Putting it after emphasizes that the watch itself is brand new.