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Dear teachers & staff
I misunderstand this topic because this is first time to learn it. It seem difficult
when do I use it ?
how can I use it ?
Is it common using in English language?
please help me !
the final request is give me some references grammar
It's to do with the style, register, etc. In different situations, different language is used- we chat to our friends in one way, but that style of language would not be appriate in other circumstances.
Asking about (Asking about appropriateness ) 
!
is give me some references grammar
Re: Asking about (Asking about appropriateness ) 
reference of grammar book ?
Linear Mode
