Je moet oppassen bij de rotonde, want de weg is glad.

Because moeten is a modal verb, like must, have to, or sometimes should in English.

With a modal verb in Dutch:

• the modal verb is the conjugated verb: moet
• the main verb stays in the infinitive: oppassen

So:

• Je moet oppassen = You must / have to be careful

You would not say je oppast here, because that would mean you are careful only in a very different structure, and oppassen normally works with a modal or as a command in this kind of warning.

Here, oppassen means to be careful, to watch out, or to take care.

In other contexts, oppassen can also mean to look after someone, especially a child:

• Ik pas op de kinderen = I’m looking after the children

But in this sentence, because it is about traffic and a slippery road, it clearly means:

• be careful
• watch out

Both je and jij can mean you.

The difference is mainly emphasis:

• je = unstressed, neutral, very common
• jij = stressed, used for emphasis or contrast

So:

• Je moet oppassen = neutral, ordinary statement
• Jij moet oppassen = you need to be careful, possibly contrasting with someone else

In normal conversation, je is the most natural choice here.

Usually, yes: je normally refers to one person in informal speech.

For plural you, Dutch usually uses:

• jullie

So:

• Je moet oppassen = You need to be careful (one person)
• Jullie moeten oppassen = You need to be careful (more than one person)

There is also formal you:

• u moet oppassen

Bij often means at, by, or near, depending on context.

In bij de rotonde, it means something like:

• at the roundabout
• near the roundabout
• when you get to the roundabout

It gives a general location.

This is different from:

• op de rotonde = on the roundabout

So:

• bij de rotonde focuses on the area/location
• op de rotonde focuses on being physically on it

In this warning, bij de rotonde sounds very natural.

Because both rotonde and weg are de-words in Dutch.

You usually just have to learn each noun with its article:

• de rotonde = the roundabout
• de weg = the road

There is not always a simple rule you can rely on, so learners often memorize nouns together with de or het.

Because want introduces a new clause giving a reason or explanation, and in Dutch it is normally preceded by a comma.

So the structure is:

• main clause: Je moet oppassen bij de rotonde
• reason clause: want de weg is glad

This is very similar to English:

• You need to be careful at the roundabout, because the road is slippery.

The comma is standard and natural here.

Because want is a coordinating conjunction, not a subordinating one.

After want, Dutch keeps normal main-clause word order:

• de weg is glad

Compare that with omdat:

• Je moet oppassen bij de rotonde, omdat de weg glad is.

So:

• want → normal word order
• omdat → verb goes to the end

Both can often be translated as because, but the grammar is different.

Yes, glad can mean smooth, but in contexts involving roads, weather, ice, or surfaces, it often means slippery.

So:

• een gladde tafel = a smooth table
• de weg is glad = the road is slippery

This is a very common Dutch use, especially in winter or in traffic warnings.

Because glad comes after is, so it is being used predicatively.

In Dutch:

• before a noun, adjectives usually take -e
• after zijn and similar verbs, they usually do not

Compare:

• de gladde weg = the slippery road
• de weg is glad = the road is slippery

So in your sentence, glad is correct because it is after is.

Moet is from moeten, which often means must or have to. But in everyday Dutch, it can also sound like strong advice, where English might sometimes prefer should.

So Je moet oppassen can feel like:

• You must be careful
• You need to be careful
• sometimes even You should be careful, depending on tone and context

In a safety warning like this, moet sounds very natural. It suggests real necessity because the road conditions are dangerous.