Quick answer
А а is the a of “father”, open and clear when stressed. Unstressed it shortens but keeps its colour.
a
How to say it
Stressed: the a of “father”. Unstressed: shorter and flatter, closer to the a in “sofa”.
А as a word
On its own, а means “and” with a twist of contrast — “whereas”, “but”: Я чита́ю, а он спит — I’m reading, whereas he’s asleep. It is the third member of the и, а, но trio and the one English has no word for.
Where О becomes А
An unstressed о is pronounced like а: Москва́ sounds like “Maskvá”, молоко́ like “malakó”. This means a lot of words contain more a-sounds than their spelling admits — and it is why knowing the stress is not optional.
Typical mistakes
Reading а as the English “ay”
the a of “father”
Russian vowels are pure, with no glide.
Using и where а is needed
Я чита́ю, а он спит.
а marks a contrast between the two halves.
Check yourself
Which word contains А?