Jag tappade min nyckelknippa i väskan och hittade den inte förrän på bussen.

In everyday Swedish, tappa is very common for accidentally losing or misplacing physical objects, especially small things like keys, gloves, or a phone.

• tappa = to drop, lose, misplace by accident
• förlora = to lose in a broader or more formal sense, such as losing a game, a war, money, or an opportunity

So for keys, Jag tappade min nyckelknippa sounds very natural.

Because nyckelknippa is singular.

Even though a keychain contains several keys, Swedish treats nyckelknippa as one single object:

• en nyckelknippa = a keychain / a bunch of keys
• min nyckelknippa = my keychain

If you were talking about the keys themselves as separate items, you would say:

• mina nycklar = my keys

It is a compound noun:

• nyckel = key
• knippa = bunch, bundle

So nyckelknippa means something like a bunch of keys, a keychain, or a set of keys, depending on context.

This is very typical Swedish word-building: two nouns are combined into one word.

That -n is the definite article attached to the noun.

• väska = a bag

• väskan = the bag

• buss = a bus
• bussen = the bus

Unlike English, Swedish usually puts the at the end of the noun instead of using a separate word.

Swedish often leaves out ownership when it is already clear from the situation.

So i väskan simply means in the bag, and the listener will often understand whose bag it is from context.

If you want to be explicit, you can absolutely say:

• i min väska = in my bag

Both are possible; i väskan is just less specific and often perfectly natural.

Here it most naturally means that the keychain ended up inside the bag.

Swedish i can sometimes cover both the idea of being inside something and moving into it, depending on the verb and context. With tappa, the idea is often that something accidentally falls and ends up somewhere.

If you wanted to make the movement more explicit, you could say something like:

• ner i väskan = down into the bag

But i väskan is completely normal here.

Because the subject is the same in both parts of the sentence, and Swedish often leaves it out in the second coordinated clause.

So:

• Jag tappade min nyckelknippa i väskan och hittade den inte ...

is a natural shorter version of:

• Jag tappade min nyckelknippa i väskan och jag hittade den inte ...

Both are possible, but omitting the second jag sounds smoother here.

Because nyckelknippa is an en-word.

In Swedish, pronouns must agree with the noun’s grammatical gender:

• en nyckelknippa → den
• ett brev → det

So after mentioning nyckelknippa, it is natural to refer back to it as den.

Inte förrän means not until.

So:

• hittade den inte förrän på bussen

means:

• didn’t find it until I was on the bus

This is a very common Swedish pattern. Förrän is normally used together with a negative idea, often with inte.

Examples:

• Jag åt inte förrän klockan tre. = I didn’t eat until three o’clock.
• Hon kom inte förrän senare. = She didn’t come until later.

Because short object pronouns often come before inte in Swedish.

So this order is very natural:

• hittade den inte

rather than:

• hittade inte den

With a full noun instead of a pronoun, Swedish more often puts inte first:

• Jag hittade inte nyckelknippan.

So the word order changes a bit depending on whether the object is a short pronoun or a full noun.

For public transport, Swedish usually uses på:

• på bussen = on the bus
• på tåget = on the train
• på planet = on the plane

Even though you are physically inside the bus, på is the normal everyday choice when talking about traveling by it.

I bussen can be used, but it sounds more like you are emphasizing the physical inside of the bus rather than the trip itself.

Yes. A close alternative is:

• Jag hittade den först på bussen.

That also means I only found it on the bus.

But inte förrän puts stronger emphasis on the fact that the keychain was not found earlier. It highlights the delay more clearly than först.