Breakdown of Я проверил подпись, которую вы вчера показали.
я
I
вы
you (plural or formal)
вчера
yesterday
показать
to show (perfective)
Questions & Answers about Я проверил подпись, которую вы вчера показали.
Why is the relative pronoun in the form которую here?
Которую is the feminine singular Accusative form of который. It needs to match the gender and number of the noun it replaces (подпись, which is feminine singular). The case, however, depends on its role in its own part of the sentence. Here, it is the direct object of the verb показали (you showed it), so it must be in the Accusative case.
Wait, подпись ends in a soft sign. How do we know it is feminine, and why didn't its ending change for the Accusative case?
Nouns ending in a soft sign (-ь) can be either masculine or feminine in Russian, and подпись (signature) happens to be feminine. While most feminine nouns change their ending in the Accusative case, feminine nouns ending in a soft sign do not change. Therefore, подпись looks exactly the same here as it does in the Nominative case.
Both проверил and показали are perfective verbs. Why?
Both verbs describe single, completed actions in the past with a clear result. You finished showing the signature yesterday, and I finished checking it. If you were repeatedly showing it or I was in the middle of checking it, we would use the imperfective verbs показывали and проверял.
Is the comma before которую strictly necessary? In English we often drop it.