Questions & Answers about Мы нашли правильный путь, которого они не видели.
Since путь ends in a soft sign, how do we know what case it is in here?
It is the direct object of нашли (found), so it is in the accusative case. путь is a unique masculine noun. Because it is inanimate, its accusative form is identical to its nominative form, staying путь. You can also spot the case and gender from the adjective правильный.
If путь has an unusual declension, why does правильный have a completely normal masculine ending?
Even though путь borrows most of its endings from feminine nouns, its grammatical gender is strictly masculine. Any adjectives or pronouns describing it will always use standard masculine endings.
Why is the relative pronoun которого used instead of the accusative который?
This happens because of the negative verb не видели (did not see). In Russian, negated verbs often force their direct objects into the genitive case rather than the accusative. This rule is called the genitive of negation. Because которого refers back to the masculine путь, it takes the masculine genitive form.
Why do we use the perfective verb нашли but the imperfective видели?
нашли (found) points to a single, completed result: the successful discovery of the path. видели (saw) is used in the imperfective to describe a continuous state in the past: they were not seeing it over that whole period of time.