For longer sentences with more verbs, it is often hard for a learner to figure out when to use the double infinitive sentence structure and when to use the past participle.
For example in the following two sentences:
They will have to pick him up from the train station. =
Sie werden ihn vom Bahnhof abholen müssen.
They would have bought it. =
Sie würden es gekauft haben.
Both these sentences contain three verbs, the first verb in the second position as normal, but then one sentence uses the double infinitive structure, whilst the second has a past participle. How do you know when to use which type of structure?
Apart from knowing that "sie würden es kaufen haben" sounds wrong and strange, I don't know how one would definitively know how to use the ge-verb form.