"He bought a book yesterday."
Translation:Anh ấy đã mua một quyển sách vào ngày hôm qua.
To sum up what many people pointed out in various comments for some questions like this one :
1 - "vào hôm qua" should be the correct and preferred/main answer. With "vào ngày hôm qua", the "ngày" is not only unnecessary but probably also grammatically wrong (can't say for sure). At the very least it's less natural and would make this sentence harder for a native Vietnamese speaker to understand.
2 - When there is an indication of time (here "yesterday") the "đã" is not needed. Same with the future "sẽ". Not sure if using those is a grammatical error or not. Vietnamese draws heavily from context (as explained in the lesson Future 1).
The second point is not really an error. It could be argued that for learning purpose and consistency with other modules, DL is right to insist on the time marking adverbs (đã and sẽ). However since drawing time from context is critical to really get vietnamese, I feel it should be taught at some point. This module would have been perfect for that since it's small, with almost no new vocabulary. I wish this module would be stricter and drop the đã, at least for the preferred answer.