Using proper conjunctions

Can you explain the difference between if and while to me? That's what I mean

1. If you are reading now I won't bother you- as I was told it is a correct sentence meaning if you are really reading now because I'm not certain I won't bother you.

2. While you are reading now I won't bother you- to me it is also correct. Just here we are certain that reading is really the case. Still, some people told that this example doesn't work with "now". Why? Is that even true?

"if" is usually used when referring to something happening at a specific time, now in your example sentence.

On the other hand, "while" tends to be used when referring to a continuous period of time. So you might say

I won't bother you while you're reading.

which means you won't bother them during the entire period of time when they're reading. You wouldn't say now with while.