Infinitives

Infinitives are the base form of verbs, which make up a significant component of a sentence. You are certainly aware of how to change an infinitive into a conjugated verb in the various tenses.
Infinitive    Conjugation    Present participle     Past participle

lachen         er lacht             lachend                       gelacht


But infinitives play a larger role than just existing as a form that can be changed by verb endings.


Future tense

An infinitive can be a part of a future tense conjugation. Although the future tense can be implied by a present tense conjugation, the actual future tense is formed from a conjugation of werden plus an infinitive at the end of the clause. Note that the singular conjugation of werden is irregular in the second and third persons:

Ich werde es lesen.                  I will read it.
Du wirst ihn besuchen.          You will visit him.
Er wird zu Hause bleiben.     He will stay home.

The plural conjugation follows a regular pattern:

Wir werden es verkaufen.               We will sell it.
Ihr werdet wieder gesund sein.     You will be well again.
Sie werden mitkommen.                 You will come along.
Sie werden morgen abfahren.        They will depart tomorrow.

Modal auxiliaries

The infinitive, however, is used in other structures besides the future tense. Significant among these is its use with modal auxiliaries:

dürfen               may,to be allowed to

können             can,to be able to

mögen               to like

müssen             must,to have to

sollen                should

wollen               to want

In the present and past tenses, the modal is conjugated and the clause ends with an infinitive:

modal auxiliary + complement + infinitive
Ich muss + zu Hause + bleiben.
I have to stay home.

Present tense
Ich muss auf ihn warten.    I have to wait for him.
Das darfst du nicht tun.      You’re not allowed to do that.

Past tense
Sie konnte ihn kaum verstehen.   She could hardly understand him.
Tina wollte Lehrerin werden.        Tina wanted to become a teacher.
Er sollte früher aufstehen.             He should get up earlier.

You should be aware that mögen has a high-frequency usage as would like, where it is conjugated in the subjunctive:

Subjunctive

Erik möchte mitkommen.  Erik would like to come along.

In the perfect and future tenses, a so-called double infinitive structure occurs at the end of the clause, with the modal auxiliary following its partner infinitive:

Sie hat es nicht lesen wollen.              She didn’t want to read it.
Das hättest du nicht sagen sollen.     You shouldn’t have said that.
Er wird nicht mitfahren dürfen.        He won’t be allowed to go along.

When a double infinitive structure occurs in a subordinate clause (following conjunctions
such as dass, weil, ob, and so on), the conjugated verb stands directly in front of the double infinitive:

Ich wusste nicht, dass sie es nicht hat        I didn’t know that she didn’t want to read it.
lesen wollen.
Sie war empört, weil du es nicht hättest    She was outraged, because you shouldn’t have
sagen sollen.                                                     said that.
Weißt du, ob er wird mitfahren dürfen?    Do you know if he will be allowed to go along?

Lassen

When the verb lassen stands alone in a sentence, its meaning is to let or leave. But lassen can be used together with an infinitive. In such a case, its meaning is to have or get. For example:

Lass den Kindern den Spaß!                     Let the children have their fun.
Ich lasse meinen Wagen reparieren.       I get my car repaired.
Ließ sie sich ein neues Kleid machen?   Did she have a new dress made?

And when it is used with the reflexive sich, lassen has a passive meaning, which in English often includes the meaning can. In this passive structure, the verbal element is again an infinitive and not a participle:

Solche Probleme lassen sich nicht leicht lösen.   Such problems are not easily solved.

Es ließ sich nicht beweisen.                                      It wasn’t proved.
Das hat sich nicht leugnen lassen.                          It couldn’t be denied.

As with the modal auxiliaries, lassen in the present and past tenses is conjugated and the

clause ends with an infinitive. In the perfect and future tenses, a double infinitive structure is

required:

◆ Present or past tense

    modal or lassen + complement + infinitive

    Ich lasse + meinen Wagen + waschen.

    I have my car washed.

◆ Perfect or future tense

    auxiliary + complement + double infinitive

    Ich habe + meinen Wagen + waschen lassen.

    I have had my car washed.