Which, that (the noun as object): noun + mà + sentence

To describe a noun with a whole sentence, like "the shirt I bought yesterday", put the sentence after the noun: cái áo em mua hôm qua. When the noun is what the action is done to, mà can join the two: cái áo mà em mua hôm qua. There's no separate word for "who", "which" or "that"; mà does all of them.

Mà is optional. In everyday speech it's usually left out: Món em thích nhất là cơm tấm. The dish I like best is cơm tấm. Keep it when the description is long, when you want to stress it, and in writing, where it's common: Đây là cuốn sách mà tôi đã nói với anh. This is the book I told you about.

Này and đó go at the very end, after the description: người em gặp hôm qua đó, that person I met yesterday.

Mà doesn't stand for the one doing the action. "The person who helped me" is người giúp em, with nothing in between; that has its own page. "The place where" and "the day when" have their own pages too. And don't use nào or ai here: they only ask questions.