Infinitives with zu
After many expressions such as versuchen (try) or es ist wichtig (it is important), use zu + infinitive. With separable verbs, zu goes between prefix and stem: aufzustehen. Modal verbs take an infinitive without zu. Um … zu expresses purpose when the subject is shared.
Understand the idea
Infinitive clauses can describe an intended action, complement another expression or state a purpose. Many require zu, but modal verbs normally take a bare infinitive. A purpose construction with um … zu generally shares its understood subject with the main clause.
Build it step by step
With an ordinary infinitive, zu appears before the verb. With a separable verb, it goes inside the word before the stem. Keep objects and other material before the final zu infinitive. If a purpose clause has its own different subject, use a full damit-clause instead. Commas are required in several important infinitive-clause patterns, including um … zu.
See it in a sentence
Ich versuche, Deutsch zu lernen.
Es ist schwer, früh aufzustehen.
A useful memory aid
Ask who performs the infinitive's action. If it is the same person as in the main clause, a purpose infinitive is often possible.
A common trap
Do not add zu after a modal, and do not separate it as zu aufstehen. The separable form is aufzustehen.