Learn English – the word for “dividing into two unequal parts”

In general, you can say things are worn out.

What do you expect? I've been using them for the past two years. They're worn out!

For many objects, there are alternative words, but they're less general:

For your car example, I'd say dilapidated. This word works best for larger, more complex objects. A house or car can be dilapidated, but the phrases ^?dilapidated keys and ^?dilapidated headphones sound weird to me. You can also say a car is broken down, but only if it doesn't actually move.

For keys, I agree that rusty works, but only if they're actually rusty. If they're worn down, I'd say they're worn.

Sometimes you don't need a special word. I might complain about my broken old headphones. You could also say they're in a state of disrepair, but it doesn't sound very conversational to me.

^? This symbol means "This phrase is of dubious acceptability. I suggest you avoid this phrase."

The two popular words for this would be "dive" (or "dove", past-tense) or wade

To "dive into the water" is close in meaning to saying "jumping into the water so as to entirely (or mostly) cover oneself in water". But to "wade into the water" generally means a slower and more gentle entry into the water, and to be incompletely submerged in the water. To wade in the water further implies walking rather than outright swimming.

So if you want to note going into the water and being completely submerged, you might say "dive under the water" instead of "dive into the water", as this makes it entirely clear that you mean complete submersion - but either is pretty clear.