Difference between using the double infinitive or past participle?

Question

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.

versus

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.

Answer 1

Your first English sentence is "will have to pick". This sentence contains also three verbs.

• will -> future -> werden
• have to -> müssen
• pick -> infinitive -> abholen

So this sentence is translated as "werden ... abholen müssen".

The other sentence is "would have bought". Again three verbs.

• would -> conditional -> würden
• have -> auxiliary -> haben
• bought -> past participle -> gekauft

That sentence is translated as "würden ... gekauft haben".

There's no much different between English and German in respect to verb forms. Fortunately, the future tenses are quite equal in both language. You only need to pay attention on the word order, but it's quite the same in both sentences as you've likely noted.

In case Wrzlprmft is right with his comment and you change would to will, you simply replace würden with werden in the German sentence as well. Note, that still the English sentences are different. It's then just Futur 1 vs Futur 2 (as they're called in German).

Answer 2

I think there's a degree of confusion about the 'double infinitive'.
The sentence:

(1) Sie werden ihn vom Bahnhof abholen müssen.

is the future of the sentence

(0) Sie müssen ihm vom Bahnhof abholen.

The future is formed with werden + infinitive. As a result the sentence (1) has nothing special, there is no 'double infinitive rule' involved here. It's just the standard form of the future.

Now if we form the present perfect of (0), what we normally expect is that müssen becomes haben + past participle of müssen (that is, gemusst). However, the actual present perfect of (0) is:

(2) Sie haben ihn vom Bahnhof abholen müssen.

In other words the expected past participle of müssen is replaced by the infinitive müssen, hence an (unexpected) double infinitive (abholen müssen).
And that is the rule of the double infinitive.

Answer 3

My impression is that this a use of "werden" to denote the passive voice and a modal verb "müssen". Together they require the double infinitive, abholen müssen.