Modal Auxiliaries and Double Infinitives

    You have already encountered the modal auxiliaries dürfen, können, mögen, müssen, sollen, and wollen in the present and past tenses. This group of verbs forms a unique conjugation in the perfect and future tenses called a double infinitive.
    When the modal auxiliary is the only verb in a sentence, it forms a participle in the perfect tenses. For example, let’s look at two modals, können and wollen.

	können	wollen
present	er kann	er will
past	er konnte	er wollte
present perfect	er hat gekonnt	er hat gewollt

   But when another verb appears in the infinitive form in the sentence, the modal does not become a participle in the perfect tenses. Instead, it appears as an infinitive side-by-side with the other infinitive, thus forming a double infinitive. This structure also occurs in the future tense. Let’s look at können and wollen once again, this time with another infinitive in the sentence. Notice the placement and formation of the double infinitive.

present	Er kann ihn einfach nicht verstehen.	He simply can’t understand him.
past	Er konnte ihn einfach nicht verstehen.	He simply couldn’t understand him.
present perfect	Er hat ihn einfach nicht verstehen können.	He simply couldn’t understand him.
past perfect	Er hatte ihn einfach nicht verstehen können.	He simply hadn’t been able to understand him.
future	Er wird ihn einfach nicht verstehen können.	He simply won’t be able to understand him.
present	Er will es kaufen.	He wants to buy it.
past	Er wollte es kaufen.	He wanted to buy it.
present perfect	Er hat es kaufen wollen.	He wanted to buy it.
past perfect	Er hatte es kaufen wollen.	He had wanted to buy it.
future	Er wird es kaufen wollen.	He’ll want to buy it.

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  In the perfect tenses, all modal auxiliaries use haben as their auxiliary. Even when the modal auxiliary is accompanied by a verb of motion, which uses sein as its auxiliary, haben remains the auxiliary in the perfect tenses, because the modal is conjugated, not the verb of motion. Compare the following pairs of sentences.

Er hat es gekauft.	He bought it.
Er hat es kaufen müssen.	He had to buy it.
Ich bin nach Hause gegangen.	I went home.
Ich habe nach Hause gehen sollen.	I was supposed to go home.

Another small group of verbs follow the double-infinitive pattern. They form a double infinitive in the perfect and future tenses just like modal auxiliaries. These verbs are helfen, hören, lassen, and sehen (help, hear, get something done, see). Compare them in the present tense and present perfect tense.

Martin hilft dem Mann arbeiten.	Martin helps the man work.
Martin hat dem Mann arbeiten helfen.	Martin helped the man work.
Hörst du die Kinder singen?	Do you hear the children singing?
Hast du die Kinder singen hören?	Did you hear the children singing?
Sie lässt ihren Wagen reparieren.	She gets her car repaired.
Sie hat ihren Wagen reparieren lassen.	She got her car repaired.
Wir sehen sie spielen.	We see them playing.
Wir haben sie spielen sehen.	We saw them playing.

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Notice that this group of verbs also uses haben as its auxiliary in the perfect tenses. Also, because helfen is a dative verb, the object in a sentence with helfen is in the dative case. For example: Martin hat dem Mann arbeiten helfen.

As the subject changes in a perfect- or future-tense sentence with a double infinitive, only the auxiliary is affected. The double infinitive remains constant. For example:

Ich habe mit ihr tanzen wollen.	I wanted to dance with her.
Du hast mit ihr tanzen wollen.	You wanted to dance with her.
Er hat mit ihr tanzen wollen.	He wanted to dance with her.
Wir haben mit ihr tanzen wollen.	We wanted to dance with her.
Ihr habt mit ihr tanzen wollen.	You wanted to dance with her.
Sie haben mit ihr tanzen wollen.	They wanted to dance with her
Ich werde ihn vom Bahnhof abholen müssen.	I’ll have to pick him up from the train station.
Du wirst ihn vom Bahnhof abholen müssen.	You’ll have to pick him up from the train station.
Er wird ihn vom Bahnhof abholen müssen.	He’ll have to pick him up from the train station.
Wir werden ihn vom Bahnhof abholen müssen.	We’ll have to pick him up from the train station.
Ihr werdet ihn vom Bahnhof abholen müssen.	You’ll have to pick him up from the train station.
Sie werden ihn vom Bahnhof abholen müssen.	They’ll have to pick him up from the train station.