"He drinks milk once a day."
अनुवाद:वह दिन में एक बार दूध पीता है।
U cannot judge it wrong for a typo. I am using a mobile device (google hindi input). I got the sentence right but a little mistake , which should be considered with typo.
I wrote पिता instead of पीता।
I was very confused over the use of articles in English but thanks to duolingo i am able to decode the 'the'.
I will be happy if i understand prepositions very well in English which has been a bone of contention to me.
Therefore my question regarding usage of preposition is "He drinks milk once in a day?"
A word can be considered as a typo if it generally doesn't lead to the formation of another valid word. But in this case "पिता" is a valid word meaning "father", so we do not accept it as a typo :)
About your other question, it doesn't matter if you leave out the "in" or if you use it. Both are correct. "He drinks milk once in a day" is also correct.