How could you let that happen?

how Adverb

— The word "how" is a question word, to ask about the way something happens or the way people do things. How will we ever solve this problem? — The word "how" is used to tell about the way something happens or the way people do things. I know how we can solve this problem? This is how to make this machine work. — used to show that you feel strongly about something. How old he looks!

how Subordinator

— You use how to tell that something happens, is done, or is true. She told him how it wasn't fun anymore.

could Verb

— If you could do something, you were able to do it. When I was younger, I could run a five-minute mile. — If you could something, but you don't know if you want to, you have the option of doing do it. She could do her math or go to the game.

you Pronoun

— Used instead of the name of the person being spoken to; it is a second-person pronoun. You are my best friend. Can I go to the shops with you? — Used to talk about anyone. You have to be 18 years old to see that film. She has problems with her eyes, and can't see you properly.

you Determinative

— Used before a word that means the person or people someone is talking to. You guys can leave now.

let Verb

— When you let someone do something, you allow them to do it. I let him come with me. — When you let a building or a space in a building, you rent it. I will let my extra room to him.

that Determiner

— Used to show which thing we are talking about; used with things that are not close to the speaker. Give me that book, not this one. Give me that, not this. That dog is hungry.

that Subordinator

— used to link a subordinate clause to a main one You said that you liked me. Give me the book that I dropped.

happen Verb

— if something happens, it starts or changes without a plan. Did anything happen at school today? It may happen to rain today.