Learn Turkish: Bahçesinde çardak olan evi satın aldık. - We bought the house that has a gazebo in its garden.

To say "there is" in Turkish, you use the word var. However, when you want to turn a "there is" sentence into a relative clause to modify a noun (like "the house THAT HAS a gazebo"), var changes into olan. So, çardak var (there is a gazebo) becomes çardak olan (that has a gazebo / with a gazebo).

Why is it bahçesinde instead of just bahçede?

The word bahçesinde contains the third-person possessive suffix -si and a buffer consonant -n- before the locative suffix -de. It literally means "in ITS garden". If you just used bahçede, it would mean "in THE garden", losing the clear connection that the garden belongs specifically to the house.

Why does ev take the accusative case here as evi?

The house is a highly specific, defined object because it is explicitly described by the relative clause (the specific house with the gazebo in its garden). Since it is the definite direct object of the verb satın aldık (we bought), it must take the accusative case ending -i.

Could we just use aldık instead of satın aldık?