
The letter 'a' is pronounced like 'u' of but, cut, shut etc.
In a word, the trailing sound of 'a' causes a lot of confusion in modern times. If it is not stressed enough, it sounds like a consonant without the vowel, like many English words put, cut, but etc. The lack of stress on trailing 't' is what would be called a 'halant' or vowel-less consonant sound.
When it is stressed, usually it ends up sounding like 'aa'.
E.g. yoga is pronounced erroneously like yogaa, whereas it is somewhat like "yo-g-uh" (NOT yogurt, but yogurt without the 'rt' sound!)
This confusion gives rise to errors of saying shiva as shivaa (pArvatI), brahma (brahman, the supreme unmanifest) as brahmaa (the creator, the manifest), drupada (the king) as drupadaa (his daughter), kRiShNa (the cowherd) as kRiShNaa (draupadI). gaNesha is pronounced as gaNeshaa. The 'sha' and 'ga' have exactly same vowel pronunciation, but mostly in English it is pronounced as 'ga' and 'shaa'. avoid such errors.
'a' is also a word in itself. The word 'a' (just the vowel), implies viShNu.
अकारो विष्णुरुद्दिष्ट उकारस्तु महेश्वरः ।
मकारस्तु स्मृतो ब्रह्मा, प्रणवस्तु त्रयात्मकः ॥a-kāraḥ viṣhṇuḥ uddiṣhṭaḥ, u-kāraḥ tu mahéshvaraḥ |makāraḥ tu smṛitaḥ brahmā, praṇavaḥ tu trayātmakaḥ || (IAST)a-kAraH viShNuH uddiShTaH, u-kAraH tu maheshwaraH |makAraH tu smRitaH brahmA, praNavaH tu trayAtmakaH || (ITRANS)