Helfen, Lassen & Senses

Question 1

Helfen/Lassen/Senses:

Answer 1

Helfen, lassen and verbs indicating the senses (such as sehen and hören) function like modal auxiliaries. Like the modals, these verbs require a double infinitive construction when forming the present perfect tense if there is a dependent infinitive involved. After verbs of this type, English often uses a present participle, but German uses a dependent infinitive.

Ich sehe ihn kommen. I see him coming.
Ich habe ihn kommen hören. I heard him coming.

Question 2

Lassen-other meanings:

Answer 2

Lassen can have different meanings depending on how it is used in the sentence. Usually, lassen means to let or allow, as in Laß den Jungen spielen! Let the boy play! (Notice that lassen takes an accusative object) But it can also mean to have something done or to have someone do something.

Wir lassen uns ein Haus bauen. We’re having a house built./We’re building a house.
Ich lasse meinen Sohn die Post abholen. I’m having my son pick up the mail.