Questions & Answers about Kiedyś dostanę ten awans.
Why does kiedyś mean 'one day' here? Doesn't it usually mean 'used to' or 'once'?
Kiedyś literally means 'sometime' or 'at some time'. Whether it refers to the past ('once') or the future ('one day') depends entirely on the verb. Because dostanę is in the future tense, kiedyś points to an unspecified time in the future.
Why use dostanę instead of będę dostawać for the future tense?
Dostanę is the future form of the perfective verb dostać (to get). We use the perfective aspect here because getting a promotion is a single, completed action, not a continuous or repeated process.
Is ten awans in the Accusative case here? Why didn't the ending change?
Yes, it is the direct object of the verb, so it is in the Accusative case. However, awans (promotion) is a masculine inanimate noun. For masculine inanimate nouns, the Accusative case looks exactly like the Nominative (dictionary) form, so both ten and awans stay the same.