[1] It typically stood in a main clause and had a subjunctive or imperative meaning; for example, it could indicate intention, e.g. índrasya nú vīryā̀ṇi prá vocam "Indra's heroic deeds will/shall I now declaim".
Ancient Greek has words that are formally similar to the Sanskrit injunctive mood, consisting of aorist and imperfect forms lacking the augment.
It is generally assumed that the augment was originally a separate particle meaning something like "then", added to indicate the past time of a form that was once mostly aspectual, and neutral with respect to tense.
Originally, its use appears to have been optional, added as necessary to clear up an otherwise ambiguous expression, similarly to time adverbs in Chinese.
The modal semantics of the augmentless forms may then be a later development within Indo-Iranian or Indo-Aryan.