Schwa and Hal are two opposite terms (concepts) in ĐevaNāgarī. Discussion about one is not complete without the other! Here we would discuss schwa.
While pronouncing English consonants we generally use trailing 'ee' (ī) or leading 'e'.
E.g. consonant 'B' is pronounced as 'bee'; 'F' is pronounced 'ef'.
While in Nāgarī, we always use trailing 'a' ('a' as in 'A'merican).
E.g. 'ब' is pronounced as 'ba'.
If you try to pronounce a single consonant (e.g. b, c, d, ...) without a trailing or leading vowel ('ee' or 'e'), you may end up in a 'natural' short vowel. This toneless (neutral) and unstressed (short) vowel is called 'schwa' in phonology; it is written precisely as 'ə' (inverted 'e') or simply as 'a'. This schwa ('a' or 'ə') is identical to vowel ‘अ’ of Nāgarī.
In the Indian Varṇamālā (alphabet), we don't have pure consonants but consonants with schwa; which are called 'Full-forms'!
Full-form = Half-form (pure consonant) + Schwa
The letter 'अ' representing the vowel 'schwa' is called Akār (अकार).
In most of the full-forms, this inherent Akār or schwa is represented by the vertical stroke. This vertical stroke is similar to the single vertical stoke of the Akār. The full-form 'ब' consists of ब् + | = ब; which is b + schwa = 'bə'; simply written as 'ba'.
This schwa gets removed if followed by -
- either a hal
- or a vowel-mark (māŧrā)
When a māŧrā (vowel-mark other than 'a') is written-after (or applied-over) a full-form, the trailing 'schwa' of that full-form is not pronounced (vanishes/gets-removed).
E.g. 'प' is a full-form 'pa';
while 'पे' is a consonant-vowel combination 'pe'.
े is a Māŧrā representing 'e'.
Here the vertical stroke becomes just a base for the applied vowel and now don't represent schwa, i.e. we pronounce just 'pe' not 'pae'.
The dotted circle in the above example is only to emphasis that a consonant is needed there. It is only visible when some typing error occurs in computers. In well-formed Nāgarī text, you won't ever see this dotted circle.
IMHO, inherent-schwa can be called 'Nihiŧ-Akār' (निहित-अकार).
* I am not a phonologist; please inform me if you find any mistake in the explanation.