Learn Russian: Я не мог не промолчать, чтобы сохранить его достоинство. - I couldn't help but stay silent to save his dignity.

The phrase не мог не literally translates to 'could not not'. In Russian, combining не with a conjugated form of мочь (to be able) and a second не before the infinitive creates a strong affirmation. It expresses that an action was inevitable, logically necessary, or impossible to resist. Because you 'could not not stay silent,' it means you absolutely did stay silent.

Why use the perfective verb промолчать here instead of the imperfective молчать?

In the не мог не construction, the choice of aspect is very precise. Using a perfective verb like промолчать indicates a single, specific action that was unavoidable in that exact moment (I was compelled to hold my tongue right then). If you used the imperfective молчать, it would describe an ongoing, continuous inability to resist an action (I couldn't help being silent in general).

Why is the verb сохранить in the infinitive after чтобы?

When the subject of the main clause ('I couldn't help but stay silent') and the purpose clause ('in order to save') is exactly the same person, Russian always uses чтобы + infinitive. If you were doing something so that someone else could save his dignity, the subjects would be different, and you would use чтобы + a past tense verb (e.g., чтобы он сохранил).

What case is его достоинство, and why doesn't его change its ending?