Causative Constructions

On its own lassen means to leave or to let. With an infinitive it becomes causative and covers three ideas: having a service done, as in Ich lasse mein Auto reparieren; giving permission, as in Ich lasse ihn gehen; and leaving someone alone, as in the imperative Lass mich in Ruhe.

Use lassen plus the infinitive of the action, with the person doing the work usually left unmentioned. Ich lasse mein Auto reparieren means I have my car repaired by a mechanic. Er lässt das Haus renovieren means he is having the house renovated. A reflexive pronoun adds a beneficiary: Ich lasse mir einen Anzug machen.

It is a strong verb with a vowel change in the second and third person singular: ich lasse, du lässt, er lässt, wir lassen, ihr lasst, sie lassen. The Präteritum is ließ, ließt, ließ, ließen, ließt, ließen. The Perfekt with a dependent infinitive uses the double infinitive, as in hat reparieren lassen.

Because lassen behaves like a modal verb here. When it governs another infinitive, its past participle gelassen is replaced by the infinitive form, producing a double infinitive: Ich habe mein Auto reparieren lassen. You only see gelassen when lassen stands alone, as in Ich habe den Schlüssel zu Hause gelassen.

Look at whether the person performing the action is named. Sie lässt sich die Haare schneiden names no agent, so it means she has her hair cut. Sie lässt die Kinder spielen names the doers, the children, so it means she lets them play. Word order and context, not the verb form, carry the difference.