Questions & Answers about Где мой зелёный ключ?
Why is there no word for "is" in this sentence?
In Russian, the present tense of the verb "to be" is almost always skipped. So, Где мой зелёный ключ? literally translates as "Where my green key?", but means "Where is my green key?".
Why do we use мой here instead of моя or моё?
The word ключ (key) ends in a consonant, which makes it a masculine noun. Possessive words like "my" must match the gender of the noun they describe, so we use the masculine form мой.
How does the ending of зелёный (green) match the noun?
The ending -ый is a standard ending for masculine adjectives in the dictionary form. Just like мой, it has to match the masculine gender of ключ. If you were talking about a feminine noun, it would change (for example, to зелёная).
How would I use the question words from this lesson (какой or чей) to ask about this key?
Because ключ is a masculine noun, any question word describing it must also take the masculine singular form. You would ask Чей это ключ? (Whose key is this?) or Какой это ключ? (Which key is this?). Notice how чей and какой match the gender just like мой and зелёный!