Questions & Answers about Это старая чёрная дверь.
Why do старая and чёрная end in -ая?
These are adjectives describing the noun дверь (door). Because дверь is a feminine noun, its adjectives must take the feminine singular ending. For most adjectives in the basic dictionary form (the nominative case), that ending is -ая.
Wait, how do we know дверь is feminine? It doesn't end in -а or -я.
It ends in a soft sign (ь). Russian words ending in a soft sign can be either masculine or feminine, so you usually just have to memorize their gender. However, in this sentence, the feminine adjectives старая and чёрная give you a big, helpful clue that дверь is feminine!
If the door is feminine, shouldn't the sentence start with the feminine Эта instead of Это?
What is the letter ё in чёрная?