Du skal krydse vejen ved lyskrydset og derefter gå op på fortovet.

Skal is the present tense of skulle, and here it expresses instruction or necessity.

In this sentence, du skal most naturally means something like:

• you need to
• you should
• you are to
• sometimes you must, depending on context

So it is stronger than a casual suggestion, but it does not always sound as harsh as English must. In directions, rules, and instructions, skal is very common.

Because they come after the modal verb skal.

In Danish, after a modal verb such as:

• skal
• kan
• vil
• må
• bør

the next verb normally stays in the infinitive, without at.

So:

• du skal krydse
• du skal gå

This is similar to English bare infinitives after modals:

• you must cross
• you can go

not you must to cross.

Because one skal can govern both verbs.

The structure is:

• Du skal krydse vejen ... og derefter gå op på fortovet

That works just like English:

• You must cross the road and then go up onto the sidewalk

You do not need to repeat skal unless you want extra emphasis or a slightly more separated structure.

So Danish often does this with coordinated verbs:

• Jeg skal spise og sove
• Vi kan komme og hjælpe

Because vejen is the definite form, meaning the road.

Danish usually puts the definite article at the end of the noun:

• en vej = a road
• vejen = the road

So instead of a separate word like English the, Danish often adds a suffix:

• common gender: -en
• neuter: -et

Here, vej is a common-gender noun, so:

• en vej
• vejen

Lyskrydset means the traffic-light intersection or, in everyday English, often simply the traffic lights.

It is a compound noun:

• lys = light
• kryds = crossing / intersection
• lyskryds = traffic-light intersection
• lyskrydset = the traffic-light intersection

This is a very Danish feature: long compound nouns are extremely common.

Also note the gender:

• et lyskryds
• lyskrydset

So the ending -et shows the definite singular of a neuter noun.

Ved here means at or by.

So ved lyskrydset means:

• at the traffic lights
• by the traffic-light intersection

It tells you the place where the crossing should happen.

Other prepositions would change the meaning:

• til lyskrydset = to the traffic lights or toward the traffic lights
• i krydset = in the intersection
• på lyskrydset would not be the normal choice here

So ved is the natural preposition for location in this phrase.

Derefter means after that, afterwards, or then.

It links the two actions in sequence:

1. cross the road
2. then go up onto the sidewalk

Yes, derefter can sound a little more formal or written than så or bagefter, but it is perfectly normal.

Rough comparisons:

• derefter = afterwards / thereafter
• bagefter = afterwards, often a bit more everyday
• så = then

In this sentence, derefter makes the order very clear.

Because op på suggests movement onto the sidewalk, not just movement along it.

This is an important difference:

• gå på fortovet = walk on the sidewalk
• gå op på fortovet = go up onto the sidewalk

Danish often uses op på when you move from a lower area to a slightly raised surface. A sidewalk is thought of as higher than the road, so op på fortovet is very natural.

So here the idea is:

• first you cross the road
• then you step up onto the sidewalk

Fortovet means the sidewalk in American English or the pavement in British English.

Again, this is the definite form:

• et fortov = a sidewalk / pavement
• fortovet = the sidewalk / pavement

So the -et ending shows that it is a neuter noun in the definite singular.

Yes, but it is very normal Danish word order.

The sentence starts with:

• Du = subject
• skal = finite verb

That gives the standard main-clause pattern:

• Du skal ...

Then the sentence continues with the infinitive phrase and another coordinated infinitive phrase:

• krydse vejen ved lyskrydset
• og derefter gå op på fortovet

So the overall structure is:

• subject + finite verb + infinitive phrase + og
  • adverb + infinitive phrase

This is a very common Danish pattern.