This page talks about when and how to use the verb "lassen," including its conjugation and what constructions it's used in.
The verb "lassen" is a special verb that can be used both as a main verb and as a modal verb (along with a second verb in the infinitive form). The meaning of "lassen" changes depending on the use.
"Sich lassen" in the third person is also an alternative way of expressing the passive voice.
Meaning "to stop" or "to quit"
Meaning "to not bring with" or "to not move something"
In this use "lassen" is combined with verbs that describe a state: stehen, liegen, sitzen,...
We often drop the 2nd verb because it’s obvious or universally known. If you do that, you have to treat "lassen" like a main verb.
„Ich lasse mein Handy zu Hause (liegen).“
Meaning "to allow something"
Meaning "something is possible / not possible"
Meaning "to arrange for something to be done" (instead of doing it yourself). It's a replacement for the passive.
The most common use of "lassen" as helping verb is a alternative way of expressing the passive voice.
The difference is that the first sentence stresses the fact that someone else is doing it (cutting her hair), while the second sentence focus on the action itself (the hair being cut).
When do we use "lassen" and when do we use "gelassen"?
If "lassen" is the main verb, it goes in the regular position of the verb (position 2 for normal sentences, position 1 for yes/no-questions). If lassen is used as a helping verb, it follows the normal rules of modal verbs.
"Lassen" is not a real modal verb because it has its own meaning and can be used without a second verb in the sentence. True modal verbs always require a second verb and have no proper meaning by themselves!