Without something happening; too … to
Ohne dass introduces a full clause describing something that does not happen. Zu … als dass expresses that a degree prevents a possible result; in hypothetical results Konjunktiv II is common. Both use verb-final clauses.
Understand the idea
A full negative accompanying clause can describe an event that does not happen. A too … for … construction can explain why another event is not possible. The intended logical relationship must be clear before choosing a connector.
Build it step by step
Use ohne dass for an action taking place without another event accompanying it. The clause has its own subject and a final finite verb. Zu … als dass presents a degree that prevents the following result, often with Konjunktiv II. With a shared subject, some meanings can also be expressed by an ohne … zu infinitive phrase.
See it in a sentence
Er ging, ohne dass ich es bemerkte.
Sie half uns, ohne dass wir sie darum baten.
A useful memory aid
Paraphrase the relationship before building the sentence: without this happening, or too much for that to be possible.
A common trap
Do not assume every negative accompanying idea needs nicht inside the ohne-dass clause. The construction already contributes the without meaning.