Bare infinitive or zu + infinitive: one short list decides

Most verbs of wishing, hoping, intending, promising, and recommending take zu + Infinitiv as their complement: "Ich hoffe, morgen früh aufzustehen"; "Er versucht, pünktlich zu sein"; "Sie hat beschlossen, Deutsch zu lernen". Adjectives in constructions like "Es ist wichtig, regelmäßig zu üben" also require zu. A simple test: if the governing verb or adjective is not in the no-zu list (modals + a small set of modal-like verbs), you almost certainly need zu.

The no-zu list has two tiers. First, the six modal verbs (können, müssen, dürfen, sollen, wollen, mögen) never take zu — "Ich kann schwimmen" (not "*zu schwimmen"). Second, the modal-like bare-infinitive verbs lassen, sehen, hören, helfen, lernen, gehen, and bleiben behave like modals in this respect: "Ich sehe sie kommen", "Er lässt das Auto reparieren", "Wir gehen einkaufen". Additionally, werden in Futur I takes a bare infinitive: "Sie wird morgen kommen". Memorise this short list and you can safely add zu after every other verb that takes a second verb.

Um … zu expresses purpose ("in order to") and requires the subject of the main clause and the infinitive clause to be the SAME person: "Ich lerne Deutsch, um in Berlin zu arbeiten" (same subject: I). damit also expresses purpose but allows a different subject in the subordinate clause: "Ich lerne Deutsch, damit meine Kinder stolz auf mich sind" (different subjects). This same-subject rule is the single most-missed point for English speakers, who use "in order to" freely regardless of subject. If in doubt: same subject → um … zu; different subjects → damit.

Ohne … zu means "without doing X" and requires the SAME subject as the main clause: "Sie hat gegessen, ohne zu bezahlen" (she ate without paying — same subject). If the subjects differ, use ohne dass: "Er ging weg, ohne dass ich es bemerkte" (he left without me noticing — different subjects). Ohne acts as a subordinating conjunction, and the infinitive goes to the end of the construction.

Statt … zu (also written anstatt … zu) means "instead of doing X" and requires the same subject as the main clause: "Er schläft, statt zu lernen" (he is sleeping instead of studying). Anstatt … zu is the more emphatic variant with identical grammar. If the subjects differ, use statt dass or anstatt dass. Like um … zu and ohne … zu, the infinitive goes to the end of the construction and the whole phrase is separated from the main clause by a comma.

With separable verbs, zu slots BETWEEN the prefix and the stem, and the result is written as a single word: auf + zu + stehen = aufzustehen; ein + zu + kaufen = einzukaufen; mit + zu + nehmen = mitzunehmen. A common mistake is writing "*zu aufstehen" (incorrect — the prefix and stem do not split around a separate zu). In a sentence: "Ich versuche, früh aufzustehen" and "Er hat vergessen, die Eltern anzurufen".

A comma is REQUIRED when the infinitive clause is introduced by um, ohne, or statt, and also when the infinitive clause has its own object or adverbial modifier: "Ich habe vergessen, dir zu schreiben" (comma required because of the indirect object dir). The 1996 Rechtschreibreform made commas optional for short bare-infinitive clauses without additional elements ("Ich versuche zu schlafen" — comma before zu is now optional). For B1 exam purposes, the safe rule is: always write the comma for any extended infinitive clause.

A bare infinitive (no zu) occurs after the six modal verbs, after lassen/sehen/hören/helfen/lernen/gehen/bleiben, with werden in Futur I, and in a few fixed expressions like "spazieren gehen" and "schlafen gehen". Zu + Infinitiv occurs after virtually every other verb, adjective, or nominal phrase that takes an infinitive complement. When in doubt, check whether the governing word is in the bare-infinitive list; if it is not, add zu.