Where (the place in which): nơi (mà)

Nơi means "place", and it also works like English "where" in "the town where I grew up". Put it after the place noun, then the sentence that describes the place: Đây là thành phố nơi em lớn lên. This is the city where I grew up. Mà can follow it for a little more weight: nơi mà em lớn lên.

Nơi can also stand without another place noun, meaning "a place" or "somewhere": Em muốn sống ở một nơi gần biển. I want to live somewhere near the sea. Hội An là nơi em thích nhất. Hội An is the place I like best.

Nơi sounds a little careful, and it's the usual choice in writing. In everyday speech people more often say chỗ (spot, place): Đây là chỗ em làm việc. This is where I work. Or they put the describing sentence straight after the noun: quán em hay ăn, the place I often eat at.

For "the day when" or "the time when", Vietnamese uses a time noun with mà or khi; that's its own pattern.