Hello ,respected sir or ma'am .My question to you is that if phrases are used just like sentence but not in a proper form , but still sometimes we can understand it so can phrase replace sentences, as sometimes in hurry people say  ''no , don't '' so we can understand it.But why do we use two means to tell the same 
- The  only thing is that sentences are appropriate  and we can understand it better .
So it should have been like if there are sentences it should be sentences only ,and if both are necesary  then what's the use of phrase?
Thanks.