Learn Russian: Это мой карандаш, а чей это ключ? - This is my pencil, but whose key is this?

Чей means "whose" and it acts like an adjective, meaning it has to match the gender of the noun it asks about. The noun here is ключ (key). Because ключ ends in a consonant, it is a masculine noun. Therefore, we use the masculine form чей.

Why is это placed between чей and ключ? Can I say это чей ключ?

Question words like чей almost always go at the very beginning of the question or clause. Russian word order is flexible, but чей это ключ? (literally "whose this is key?") sounds the most natural. Here, это acts as the unchanging word for "this is".

Why does the English translation say "but", but the Russian sentence uses а instead of но?

Russian has two words for "but". Но is used for a strong contradiction or unexpected twist. А is used for a softer contrast or when comparing two things, like pointing out your pencil versus someone else's key. When you switch focus from one object to another, а is the right choice.

Both карандаш and ключ are masculine. How can we tell just by looking at them?