In Russian, an unaccent o is pronouce like an a. How can we know whtere the letter is unaccent or not ?

Answer 1

You have to know how to properly pronounce the word. If the syllable is stressed then it is accented such as "Sobaka" - dog. 'ba' is stressed, not 'so' therefore you would pronounce the word as 'sabaka'
In the word 'koshka' - cat 'ko' is stressed that's so you would pronounce it as written 'koshka'

I hope this helps

Answer 2

all you need to know that you pronounce "o" as "o" in stressed positions and as "a" in unstressed. Stress is always shown in good dictionaries, 'cause there is no rule about stress. Plus, there are some words that spelled the same way but according to stress mean different things as: 'zamok (stressed on first syllable and pronounced as zamak), mean castle and za'mok (pronounced as zamok) mean lock.

Answer 3

You pretty much can't. That's one of the reasons why Slavic languages are so much fun.