Learn French: Ils ont skié sur une piste difficile. - They skied on a difficult slope.

Yes, it is! Skier is a regular -er verb. To form the past tense here, you use the helping verb avoir (ils ont) and change the -er ending to -é, giving you skié.

How would I add that they skied on this slope 'for' two hours?

You would add pendant deux heures to the end of the sentence. As you are learning in this lesson, pendant is used to mean 'for' when talking about a set, closed period of time, making it perfect for a completed past action like this one.

Why do we use the preposition sur here?

In French, you ski 'on' a surface like a slope (sur une piste), which matches English nicely. If you were talking about skiing in the mountains in general, you would instead use à (à la montagne) or dans (dans les Alpes).

Does piste only mean 'slope'?

Not always! Piste generally means a track, trail, or path. Depending on the context, it can mean a ski slope (une piste de ski), a dance floor (une piste de danse), a dirt trail, or even an airport runway.