I am always unsure when to use ~ and ? in Vietnamese. Also, modern Vietnamese keyboard software has this thing called "New Accent Rule" (or "Bỏ Dấu Kiểu Mới"). The following attempt to address these two issues and has links to more advanced topics for readers who's interested in learning more.

When to use ~ and when to use ?
There are exceptions but a quick rule of thumb for double-words (từ láy) is:
1. Use ~ if the other word has the ` accent (huyền) or . (nặng) accent. For example: tầm tã, lững lờ, vội vã, gọn ghẽ.
2. Use ? if the other word has no accent (thanh ngang) or ' (sắc) accent. For example: dư dả, lửng lơ, nóng nẩy, vất vả.

To help you remember this simple rule but cover many spelling error cases, remember the following poem:


Where to put the accent marks?
As you might have notice, in this forum, if you checked "Bỏ dấu kiểu mới" and typed in "Thúy Nga", it will automatically convert to "Thuý Nga". Other similar cases are "Hòa Bình" and "Hoà Bình" Why??? I don't know the history of Vietnamese enough and hence not certain which way is correct. However, I know why the "new accent" created weird-looking words like the above and can say for sure that this is not caused by a bug in the Vietnamese Keyboard software used in this forum. The following rule of precedence is used in the "new way", pay attention to rule #3:

Trong Notepad phải lựa unicode font. Vào đổi font thành Tahoma thì được. Nếu dùng Windows cũ trước Windows XP thì botay.com.