lassen

One verb, three everyday meanings. The form is the same every time; the context tells you which is meant.

First, the forms: ich lasse, du lässt, er/sie lässt, wir lassen. The Perfekt with a second verb is hat … lassen, not gelassen.

Now the important one. Ich lasse mein Auto reparieren does not mean you repair it. It means somebody else does, and you arranged it. German uses this constantly.

• Ich lasse mein Auto reparieren.

  I am having my car repaired.

  the mechanic does the work; the infinitive goes to the end

• Ich lasse meine Haare schneiden.

  I am having my hair cut.

• Wir haben die Heizung reparieren lassen.

  We had the heating repaired.

  Perfekt: hat/haben + Infinitiv + lassen

Compare: Ich repariere das Auto — I do it myself. Ich lasse das Auto reparieren — somebody does it for me.

The second meaning is letting. Here a person is being allowed to do something, so there is someone in the accusative.

• Lass mich erklären.

  Let me explain.

  imperative; mich is the person doing the explaining

• Meine Eltern lassen mich das Auto benutzen.

  My parents let me use the car.

• Lass uns morgen früher anfangen.

  Let's start earlier tomorrow.

  lass uns is the ordinary way to make a suggestion

The third meaning is the plainest: leaving something somewhere. No second verb, so it is easy to spot.

• Ich lasse die Tasche hier.

  I am leaving the bag here.

  no infinitive at the end — that is the clue

• Ich habe mein Handy zu Hause gelassen.

  I left my phone at home.

  here the Partizip II really is gelassen

With a second verb the Perfekt is … reparieren lassen. With no second verb it is … gelassen.