英文幾時要發r音幾時唔使?

It has different pronounciation for different words, there isn't any specific rule for this.
All you have to do is try to listen to English people speaking. Eg, BBC, CNN, and read more. Read it aloud. And finally use them. Compose some compositions, and all you need is a pen and a paper.

字尾R 在美式英語中會捲脷， 英式英語則不會。 例：

butter 美：butrrrr； 英：but a
water 美：whatrrrrr； 英： what a