Learn Turkish: Kuş bahçede. - The bird is in the garden.

The locative suffix changes based on vowel harmony. If the last vowel of the word is a front vowel (e, i, ö, ü), we use -de. Since the last vowel in bahçe is e, it becomes bahçede. If the word had a back vowel (a, ı, o, u), like araba (car), we would use -da (arabada).

Does -de specifically mean "in"? Could it mean "at the garden"?

The suffix -de (and its other forms) is a general location marker that covers "in", "on", and "at". We choose "in" for the English translation because that's how we normally talk about gardens, but in Turkish, bahçede simply means the bird's location is the garden.

There is no verb in this sentence! Where is the word for "is"?

Turkish doesn't use a separate verb for "is" in simple statements like this. Just placing the subject (Kuş) and the location (bahçede) next to each other is enough to mean "The bird is in the garden."

Where is the word for "the" in this sentence?

Turkish does not have a separate word for "the". A word like kuş can mean "a bird", "birds" in general, or "the bird". We usually know it means "the bird" from the context of the conversation.