This is the 8th most frequent Vietnamese word.
này
"This" or "these" (used to indicate something close in distance or time).
Here, 'này' specifies which object (the book) is being referred to, emphasizing immediacy or specificity.
Cuốn sách này rất hay.
This book is very interesting.
'Này' in this sentence refers to a specific exercise among potentially several others, indicating the particular one being discussed.
Bạn có thể làm bài này được không?
Can you do this exercise?
In this context, 'này' is used in 'ngày hôm nay' to emphasize the current day, as opposed to other days.