Als je knie blijft bloeden, moet je er snel een pleister op doen.

Because blijven + infinitive is a fixed verbal pattern in Dutch.

• blijven = to keep / to continue
• bloeden = to bleed

So blijft bloeden means keeps bleeding or continues to bleed.

In Dutch, when you use a conjugated verb plus another verb in the infinitive, the finite verb usually comes first and the infinitive comes later:

• je knie blijft bloeden
• literally: your knee stays bleeding
• natural English: your knee keeps bleeding

So bloedt blijft would be ungrammatical here.

Because the sentence begins with a subordinate clause:

• Als je knie blijft bloeden = If your knee keeps bleeding

When a Dutch main clause comes after something placed first in the sentence, Dutch uses inversion: the verb comes before the subject.

Normal main clause order would be:

• Je moet er snel een pleister op doen.

But after the als clause comes first, it becomes:

• Als je knie blijft bloeden, moet je er snel een pleister op doen.

So this is a very common Dutch pattern:

• first element
• then finite verb
• then subject

Here als means if.

• Als je knie blijft bloeden = If your knee keeps bleeding

In many everyday contexts, als can sometimes feel close to when, especially in practical instructions, but the standard meaning here is conditional: if this happens, then do that.

For learners, the safest translation in this sentence is if.

Because je is the normal unstressed possessive adjective meaning your.

• je knie = your knee

Dutch often prefers je in ordinary sentences.
jouw is the stressed form and is used for emphasis, contrast, or clarity.

Compare:

• Je knie bloedt. = Your knee is bleeding.
• Jouw knie bloedt, niet de mijne. = Your knee is bleeding, not mine.

So in this sentence, je knie is the natural choice.

Here er refers back to the place where the plaster goes — in this case, the knee or the wound on it.

Dutch often uses er with prepositions when English might just use it or leave it understood.

• een pleister op iets doen = to put a plaster on something
• that iets becomes er ... op
• so: er een pleister op doen = put a plaster on it

This is a very common Dutch structure:

• Ik leg het boek op de tafel.
• Ik leg het boek erop. = I put the book on it / on there.

So in your sentence, er ... op works together.

Because opdoen is a separable verb here.

The full verb is:

• opdoen = to put on / apply

In a main clause, separable verbs split apart:

• Je doet een pleister op.

With extra words added, the parts stay separated:

• Je moet er snel een pleister op doen.

At the end, when verbs gather together, you often see the infinitive part:

• ... een pleister op doen

So op belongs with doen, even though it is written separately in this sentence.

Een pleister op doen is a natural, everyday way to say put a plaster on.

Other possibilities exist:

• een pleister aanbrengen = more formal, like apply a plaster
• een pleister plakken = also common, literally stick a plaster on

So:

• Doe er snel een pleister op = very natural spoken Dutch
• Breng snel een pleister aan = more formal
• Plak er snel een pleister op = also very common

A learner should recognize all of them, but the sentence you were given is perfectly normal.

Pleister means the small sticky medical strip you put over a cut.

In English, that is:

• plaster in British English
• Band-Aid or adhesive bandage in American English

So Dutch pleister is the general word, not necessarily a brand name.

Because snel is an adverb meaning quickly, and in Dutch adverbs often appear before the object phrase they modify or before the final verb cluster.

So:

• moet je er snel een pleister op doen

This is a natural word order meaning:

• you should quickly put a plaster on it

You may also hear slightly different word orders in speech, but this one is standard and natural.

Yes, moet is from moeten, which often means must, have to, or should, depending on context.

Here it sounds like practical advice or instruction:

• moet je = you should / you need to / you must

In English, must can sound quite strong, so a natural translation may be:

• you should quickly put a plaster on it
• or you need to quickly put a plaster on it

The exact English wording depends on tone, but the Dutch is very normal.

Actually, there are two je forms in the sentence, but they do different jobs:

• je knie = your knee → possessive je = your
• moet je = you must / you should → subject pronoun je = you

Dutch uses the same form je for both:

• possessive: your
• subject pronoun: you

That can feel confusing at first, but native speakers distinguish them automatically from the sentence structure.

Not usually if you want the meaning if your knee keeps bleeding.

• als = if
• wanneer = when

Wanneer je knie blijft bloeden sounds more like when your knee keeps bleeding, which suggests the situation is expected rather than conditional.

For a practical instruction based on a possible situation, als is the right choice.

It can be used for both.

• Mijn vinger bloedt. = My finger is bleeding.
• Zijn knie bloedt. = His knee is bleeding.

So bloeden is the normal verb whether it is a small scrape or something more serious. In this sentence, because it mentions a pleister, the context suggests a minor wound.