I understand the difference between these two approaches(prescriptive and descriptive?), but I need to know where or how do we use each them?
When is prescriptive is better to use?
This is quite a broad question. But there are some easy answers for when 'prescriptive grammar' (more specifically, grammar as prescribed by grammar books) should be used:
- When teaching English to non-native speakers
- When learning English as a non-native speaker
- When writing English, except in messages to friends, relatives etc.
- In any professional context
- In public speaking
You can't use descriptive grammar as such. Instead, descriptive grammar is the name for a non-judgemental approach to grammar. Academic linguists use this approach, because they see themselves as scientists (or at least empiricists), rather than being concerned with upholding norms. (At least, that's the story told to beginning linguistics students ;-) )