Правопис голосних: акання і якання на письмі

The principle

Russian spelling is morphological: вода and воды keep the same о, and you work out the pronunciation from the stress. Belarusian spelling is phonetic: it writes the vowel that is actually said. This single difference is behind most of the spelling errors a Russian speaker makes.

Akanne

Unstressed о and э are written а:

Jakanne

Unstressed е is written я, but only in the syllable immediately before the stress. Further away it stays е:

• лес → лясны - one syllable before the stress, so я
• лес → лесавік - two syllables away, so е
• сем → сямнаццаць

This is the rule people get wrong most often, and the fix is mechanical: count syllables back from the stress. One syllable, я. More, е. Two exceptions in the 2008 rules: е stays after г, к, х (герой, кефір), and the particle не and the preposition без always keep е (не быў, без меры).

What stays put

• Borrowings give a few exceptions learned as words: радыё keeps its ё. A final unstressed о became а under the current rules - трыа, Токіа; трыо and Токіо survive only in dictionaries printed before them.
• Roots with ро, ло that alternate with ры, лы escape akanne: кроў - крыві, дровы - дрывотня, гром - грымець (2008 rules, §4).
• ы and і never alternate this way.

Why it matters beyond spelling

Because the writing follows the sound, one root can look like three: сэрца - сардэчны - сярдзіты. Recognising that they are one family is a reading skill, not just a spelling one.