It all comes down to the difference between a and ă. A is pronounced like a in father and ă is like a in about (the sound of ă is schwa, ə, actually the most common sound in English).

You may also want to listen to the pronunciation of the indefinite form căpşună on Forvo: https://ro.forvo.com/search/c%C4%83p%C5%9Fun%C4%83/.