Het is handig om je sleutel in je jaszak te bewaren.

Here het is a dummy subject, like it in English in It is useful to... or It is convenient to....

It does not refer to a specific thing. The real content of the sentence is the infinitive clause:

om je sleutel in je jaszak te bewaren

So the structure is basically:

Het is handig + [to keep your key in your coat pocket]

This is a very common Dutch pattern:

het is + adjective + om ... te + infinitive

It is used for sentences like:

• Het is moeilijk om dit te doen.
• Het is fijn om thuis te werken.
• Het is handig om je sleutel in je jaszak te bewaren.

In English, we usually just say to keep. In Dutch, that idea is often expressed with om ... te + verb.

Also, om here does not strongly mean in order to. It is mainly part of the normal infinitive construction after handig.

Because Dutch usually puts the infinitive at the end in this kind of clause.

So in:

om je sleutel in je jaszak te bewaren

the order is:

• je sleutel = object
• in je jaszak = location
• bewaren = infinitive verb at the end

This is very normal Dutch word order, even though it feels different from English.

Both instances of je mean your here:

• je sleutel = your key
• je jaszak = your coat pocket

So in this sentence, je is a possessive determiner.

Dutch je can also mean you, but not in this sentence, because it is directly in front of nouns.

Both can mean your.

• je is the normal, unstressed form
• jouw is the stressed form

So:

• je sleutel = neutral, ordinary
• jouw sleutel = your key, with emphasis or contrast

For example:

• Waar is je sleutel? = Where is your key?
• Ik bedoel jouw sleutel, niet die van mij. = I mean your key, not mine.

In the original sentence, there is no contrast, so je is the natural choice.

Yes, it could also be plural.

The singular je sleutel sounds natural if you mean:

• one important key, such as a house key, or
• a key in a general, typical sense

If you mean several keys, you can say:

Het is handig om je sleutels in je jaszak te bewaren.

Both are correct; it just depends on what you mean.

Because Dutch writes compound nouns as one word.

• jas = coat / jacket
• zak = pocket / bag

Together:

jaszak = coat pocket / jacket pocket

This is very normal in Dutch. English often writes these as separate words, but Dutch usually combines them:

• huisdeur = house door / front door
• keukentafel = kitchen table
• treinstation = train station

It means specifically a coat pocket or jacket pocket.

If you want a different kind of pocket, Dutch often uses a different compound:

• jaszak = coat/jacket pocket
• broekzak = trouser pocket
• binnenzak = inside pocket

So Dutch is often more specific here than English pocket.

Here handig means something like:

• convenient
• practical
• useful

It does not mean skilful here.

That is important because handig can also describe a person:

• Zij is handig. = She is skilful / good with her hands.

But in your sentence, it describes the situation, so practical or convenient is the right idea.

Bewaren means something like keep, store, or keep in a safe/known place.

That makes it a good choice here, because the sentence is about where it is practical to keep the key.

Compare:

• bewaren = keep/store
• stoppen = put/stick something somewhere
• houden = keep/hold

So:

• je sleutel in je jaszak bewaren = keep your key in your coat pocket
• je sleutel in je jaszak stoppen = put your key into your coat pocket

The original sentence is about the habit or storage place, not the single action of putting it there.

You can sometimes move parts around in Dutch, but the original order is the most natural here:

om je sleutel in je jaszak te bewaren

That order sounds smooth and standard because the location phrase comes before the final infinitive.

A version like:

om je sleutel te bewaren in je jaszak

would still be understandable, but it sounds less natural in neutral Dutch.

Yes. A formal version would be:

Het is handig om uw sleutel in uw jaszak te bewaren.

That is grammatically correct.

But the original with je sounds more natural for general advice, because Dutch often uses informal je the way English uses general you. So the sentence with je is the most natural everyday version.