Where does the official grammar stands on i vs y? thế kỷ and thể kỉ both work?
Yes, they both work.
I and Y both represent the sound of /i/ in Vietnamese. But there are still many arguments about how to write them.
There are two ways to write words which have this structure:
Consonants h, m, l, k, t and qu + the vowel /i/.
The 1st way:
- If that word is a native Vietnamese word: use i for it (E.g: ki bo (mean/skimpy), mi mắt (eyelid)).
- If that word is a Sino-Vietnamese word: use y for it (E.g: công ty (company), vật lý (physics), thế kỷ (century)).
- Always use y after qu regardless that is a native Vietnamese or Sino-Vietnamese word (E.g: quỷ (demon), quỳ (kneel), quý (precious)).
- Use i for remaining consonants (b, d, đ, g, v, s, gi, nh, etc.) (E.g: vì (because), đi (go)).
The 2nd way:
Always write i regardless that is a native Vietnamese or Sino-Vietnamese word (E.g: công ti, lí thuyết, thế kỉ, ki bo, mi mắt). But after qu, we still write y (E.g: quỷ, quỳ, quý).
In this course, we use the 1st way. But you can use any way that you like because people can understand both of them.
For other words that contain i/y but don't have the same structure as above (initial consonants + vowel /i/), you must follow what the course teaches you because there is only one way to write them (E.g: yếu (weak/important)).