'Why you did that' and 'Why did you do that' difference [duplicate]

I asked one person 'Why you did that' referring to something he did in the past, and he responded something like 'Why are you saying that, are you a little kid?'. I was confused and asked him why would he say that, he responded that I need to use 'Why did you do that' instead.

Why 'Why you did that' was incorrect and why he was referring to a little kid thing?

Consider the statement you do that. In order to turn this into a question in the past tense, you must add the auxiliary did to the original statement, to form the question why did you do that?.

Children learning to speak a language have to construct a grammar from the language that they hear around them. They do not start out speaking perfectly, and often make mistakes like why you did that because that is the extent to which their grammar models the adult grammar. I'm sorry that this person was so rude to you in your endeavors to speak English. Don't be afraid to make mistakes and keep asking questions and you'll get the hang of it!

"Why you did that" is wrong merely because of the sentence structure. The other guy is right to correct it to "why did you do that". You must state the tenses clearly and somewhat early. It's like saying "He ran had before" when really it's "He had ran before."