hi..i am hindi speaker...
kuch na kaho = dont say anything...but when it is used in song or some poem , it shows soft emotions..not harsh
kehna= tell..... so bolna and kehna are used in different way as in english speak and tell are used
2. sub kuch = everything, anything = kuch bhi
3. na and mat are same like dont...
hindi is national language in india..but in fact the national language is pure form of hindi..there are too many regional hindies..
so in some region people say "na kar" while in another regions people say "mat karo" both means "dont do"
and nahin means not....i am not a policman = mein policman nahin hu
well, i am happy to know that you are learning hindi, i am medicine student,indian citizen. pleased to help you!