We use the construction ohne … zu when we want to limit or negate something.
Karsten geht nach Hause, ohne ein Wort zu sagen. Karsten goes home without saying a word.
Mike hat zwei Monate Urlaub gemacht, ohne viel Geld auszugeben. Mike went on holiday for two months without spending a lot of money.
Ohne ... zu is an infinitive construction (Infinitivkonstruktion). Infinitive constructions are made with zu + infinitive.
Even though the name might seem to imply that they are something complicated, they are really easy.
We just have to bear two things in mind:
  • The subject in the subordinate clause is the same as in the main clause.
  • We use the infinitive form with zu at the end of the sentence.
Let's look at an example:
Karsten ist einfach gegangen, ohne sich zu verabschieden. Karsten simply left without saying goodbye.
Karsten left.
+
Karsten did not say goodbye.
Subject in main clause
=
Subject in subordinate clause
...zu verabschieden
Infinitive with zu at the end of the sentence
That's it! Easy as that :)
Don't forget: if a verb is separable, zu goes sandwiched between the prefix and the main part of the verb!
→ Mike hat zwei Monate Urlaub gemacht, ohne viel Geld auszugeben.