Laat je arm zakken als hij pijn doet.

Here, laat is the imperative (command form) of laten.

In this sentence, laten + infinitive means something like:

• let ...
• have ...
• make ...

So Laat je arm zakken literally means Let your arm sink/drop, and more naturally in English: Lower your arm or Put your arm down.

It is not the adjective laat meaning late.

In je arm, je means your.

That is because je comes directly before a noun, arm. So here it is a possessive determiner.

Compare:

• je arm = your arm
• ik zie je = I see you

So although je can mean you, in this sentence it means your.

Also, the command itself is directed at you implicitly, because Dutch imperatives usually leave out the subject:

• Laat je arm zakken. = Lower your arm.

Because it comes after laten.

Dutch often uses this pattern:

• laten + infinitive

So:

• je arm laten zakken = to lower your arm
• literally: to let your arm go down

That is why you get:

• Laat je arm zakken

not a conjugated form like zakt.

Zakken means to sink, to drop, to go down, or to lower depending on context.

In this sentence, it suggests movement downward:

• laat je arm zakken = lower your arm / let your arm drop down

It is a very natural verb here because an arm is physically moving downward.

Als has several meanings in Dutch. In this sentence, it means if or when.

So:

• als hij pijn doet = if it hurts / when it hurts

For learners, it is often easiest to remember that als can introduce a condition or situation, not just a comparison.

Examples:

• Bel me als je thuis bent. = Call me when you’re home.
• Als het regent, blijven we binnen. = If it rains, we stay inside.

Here, both if and when can fit depending on the context.

Hij refers to je arm.

In Dutch, nouns can be referred to with pronouns based on their grammatical gender. Arm is a de-word:

• de arm

A singular de-word is commonly referred to with hij.

So:

• je arm ... hij
  = your arm ... it

Even though English uses it for things and body parts, Dutch can use hij for a de-word noun.

Because arm is a de-word, not a het-word.

A useful basic rule is:

• singular de-word → often referred to with hij
• singular het-word → often referred to with het

So:

• de arm → hij
• het been → het

That said, in everyday Dutch, people do not always follow this in a very strict, formal way in all contexts, but in a sentence like this, hij for de arm is completely normal.

Pijn doen is an idiomatic expression meaning to hurt.

So:

• Mijn arm doet pijn. = My arm hurts.
• Doet het pijn? = Does it hurt?

Literally, it looks like does pain, but you should learn it as a set expression.

In your sentence:

• als hij pijn doet = if it hurts / if your arm hurts

Because after als, you have a subordinate clause, and in Dutch subordinate clauses, the conjugated verb usually goes to the end.

So:

• main clause: Hij doet pijn.
• subordinate clause: als hij pijn doet

This is a very important Dutch word-order pattern.

Compare:

• Het been doet pijn. = The leg hurts.
• ... als het been pijn doet. = ... if the leg hurts.

It is a natural way to say lower your arm.

Literally, it is something like:

• Let your arm go down

But in real usage, it simply means:

• Lower your arm
• Put your arm down

Dutch often uses laten in this kind of causative or permissive structure where English might just use a simple verb.

Not in exactly the same way.

Als works well for if/when in a conditional sense:

• Laat je arm zakken als hij pijn doet.

Wanneer means when, but it sounds more specifically time-related, and less like a practical condition. In an instruction like this, als is more natural.

Also, wanneer je arm pijn doet would repeat je arm, while hij smoothly refers back to it.

So the original sentence sounds more natural for advice or instruction.

Yes, absolutely.

Both are natural:

• Laat je arm zakken als hij pijn doet.
• Laat je arm zakken als je arm pijn doet.

Using hij avoids repeating je arm, so it sounds a bit smoother.

Using the noun again is also clear and common, especially for learners or in contexts where you want to avoid ambiguity.

Yes. Je is the informal your.

If you wanted a more formal version, you could use uw:

• Laat uw arm zakken als hij pijn doet.

That said, in medical or instructional contexts, Dutch often uses je quite freely, depending on the relationship and tone.

A rough learner-friendly pronunciation would be:

• Laat yuh arm ZAK-kuh(n) als hiy payn doot

A few points:

• laat has a long aa sound
• je is usually weak and short, like yuh
• zakken has stress on the first syllable: ZAK-
• g in standard Dutch is usually a throaty sound, but in zakken there is no g sound, just k
• pijn sounds roughly like pine
• doet has a long oe sound, like boot but shorter and with Dutch vowel quality

Yes, very often.

Examples:

• Mijn rug doet pijn. = My back hurts.
• Mijn knie doet pijn. = My knee hurts.
• Doet je schouder pijn? = Does your shoulder hurt?

So pijn doen is one of the most useful expressions to learn for talking about discomfort or pain.

Yes, a few things:

1. je does not mean you here; it means your.
2. laten + infinitive is more common in Dutch than a direct English-style verb choice.
3. hij refers to arm, even though English would say it.
4. pijn doen is idiomatic and should be learned as a chunk: to hurt.
5. In the als clause, the verb goes to the end: als hij pijn doet.

So even though the sentence is short, it contains several very common Dutch grammar patterns.