Breakdown of Они встретились за час до ужина.
они
they
час
hour
встретиться
to meet / to meet up (with each other)
до
before, until
Questions & Answers about Они встретились за час до ужина.
How does the за ... до structure work in this sentence?
To express how long before an event something happened, Russian uses the formula за + amount of time in Accusative (час) + до + the event in Genitive (ужина). You cannot simply say час до ужина to mean 'an hour before dinner'.
Why does час look like it is in the nominative case if за requires the accusative here?
Час is a masculine inanimate noun. For inanimate masculine nouns, the accusative form is identical to the dictionary (nominative) form, so the word does not change its ending.
Why do we use the reflexive verb встретились instead of just встретили?
The reflexive ending -сь makes the action reciprocal here, meaning they met each other. If you used встретили, it would mean they met someone else, and you would need to add a direct object to say who they met.
Why does ужин change to ужина at the end of the sentence?
The preposition до, which means 'before' or 'until' here, always takes the genitive case. Because ужин (dinner) is a masculine noun ending in a consonant, we add -а to form the genitive.