Is there an algorithm help us to write each even number as sum of primes numbers? 1) Is there an algorithm help us to  write each even number as sum of primes numbers :
for example :$4=2+2$
$8=3+5$
where : $3,5,2$ are primes 
2) why we couldn't writing all odd numbers as sum of primes?
Late Edit: example look :$7=3+2+2$ 
I would be interest for any replies or any comments

Re: Original question, under the assumption that we are speaking of the sum of two primes...
Your first question is indeed Golbach's conjecture (addressing your comment below). It is an open question (as to whether every even number greater than $2$ can be written as the sum of two primes).
For your second question.
All prime numbers, with the exception of $2$, is odd. And odd + odd = even.
Some odd numbers can be written as the sum of two primes: $2+3 = 5$, $\;2+5 = 7, \;2+7 = 9,\;2+11 = 13,  2+ 17 = 19$. 
But in general, this won't work for all odd numbers.  
For example, $11 = 1+10 = 2+9 = 3+8 = 4+7 = 5+6$. All fail to be sums of two prime numbers. $17 = 1+16 = 2+15=3+14 = 4+13 = 5+12=6+11 = 7+10 = 8+9$. Again, $17$ (an odd number) fails to be the sum of two primes.
Adding two odd numbers (and so adding two odd primes) will always result in an even number.
