Learn Spanish (Spain): El espía pasó inadvertido sin que nadie lo viera. - The spy went unnoticed without anyone seeing him.

The phrase sin que (without) always triggers the subjunctive in Spanish because it introduces an outcome that did not happen or was prevented. Since the main verb pasó (went) is in the past tense, the dependent clause must align with it by using the imperfect subjunctive viera.

Could I just use the infinitive and say 'sin ver a nadie'?

No, because there is a change of subject in this sentence. The first subject is el espía (the spy) and the second is nadie (nobody). When the subject changes, you must use sin que + subjunctive. If the spy was the subject of both actions (e.g., 'The spy went unnoticed without seeing anyone'), you would then use sin + infinitive: El espía pasó inadvertido sin ver a nadie.

Since 'nadie' means 'nobody', why does the English translation say 'without anyone'?

In Spanish, the word sin (without) creates a negative environment. When you want to express 'without anyone' or 'without anything', Spanish requires the negative pronouns nadie (nobody) or nada (nothing) to match that underlying negative context. Saying sin que alguien would sound very unnatural.

Could I say 'pasó desapercibido' instead of 'pasó inadvertido'?

Yes, absolutely. Both pasar inadvertido and pasar desapercibido are extremely common ways to say 'to go unnoticed' in Spain. While traditional grammarians used to argue that desapercibido technically means 'unprepared', modern Spanish speakers use both expressions completely interchangeably.