Breakdown of Он пошёл на собрание в качестве заместителя.
он
he / it (for a masculine noun) / it (for a neuter noun)
в
in / to (direction)
на
on, at / by (transport) / to (a place) / to (in answer to)
пойти
to go (perfective, on foot) / to start (of rain, snow)
Questions & Answers about Он пошёл на собрание в качестве заместителя.
Why is заместителя in the Genitive case here?
The phrase в качестве translates to "in the capacity of" or "as," and it must always be followed by the Genitive case. Since заместитель is a masculine noun ending in a soft sign, its Genitive form is заместителя.
If I wanted to use как (as) instead of в качестве, what case would the word for "deputy" be in?
If you use как, the noun must agree with the case of the person it refers to. Here, "he" (он) is the subject in the Nominative case, so you would say как заместитель (Nominative). Both в качестве заместителя and как заместитель are natural ways to say "as a deputy."
Why do we use на собрание (Accusative) rather than на собрании (Prepositional)?
Because the verb пошёл (went) indicates motion towards a destination. When expressing direction ("to a meeting"), the preposition на takes the Accusative case. If you wanted to say something happened at the meeting (a fixed location), you would use the Prepositional case: на собрании.
Is there a trick to remembering the word заместитель?
Yes! Look at the root: it is related to место (place). A заместитель is literally someone who takes the place of another person, hence a deputy, substitute, or acting manager.