Learn Dutch: Het is erg druk op de weg. - It is very busy on the road.

While the noun de druk means 'the pressure', the adjective druk means 'busy' or 'crowded'. In Dutch, you use druk to describe heavy traffic, busy places, or even a busy schedule.

Can I use heel instead of erg in this sentence?

Yes! Heel druk and erg druk both mean 'very busy' and are completely interchangeable here. Note that erg can also mean 'terrible' or 'bad' on its own, but right before another adjective, it just means 'very'.

Why is it op de weg instead of in de weg?

In Dutch, you drive 'on' the road (op de weg), just like in English. If you say something is in de weg, it means it is literally 'in the way' or blocking your path!

Since this lesson is about omdat, how would the word order change if I put omdat before this sentence?

The word omdat (because) pushes the conjugated verb to the end of its clause. To say 'because it is very busy on the road', the verb is moves to the very end, making it: omdat het erg druk op de weg is.