Learn German: Lass den Schlüssel sofort nachmachen! - Have the key duplicated immediately!

When using lassen to mean having something done by someone else, German always pairs it with an active infinitive at the end of the sentence. Even though the key is receiving the action, you do not use a past participle like nachgemacht. The structure is simply lassen + object + infinitive.

What is the difference between saying Lass den Schlüssel nachmachen and Mach den Schlüssel nach?

Mach den Schlüssel nach! means you are telling the person to duplicate the key themselves. By adding Lass, the sentence becomes causative. It means you want them to arrange for a third party, like a locksmith, to do the actual work.

nachmachen is a separable verb. Why is it not separated in this sentence?

Because lassen acts similarly to a modal verb here. The conjugated verb Lass takes the first position for this command, pushing the second verb to the very end of the sentence in its full, unseparated infinitive form. You only separate a prefix when the verb is the conjugated main verb.

Why is the first verb conjugated as Lass without an umlaut?