-◌ा → -◌े (Direct / Oblique case)
Nouns change form before postpositions — direct case for subjects, oblique case before को, में, से, etc.
Example
लड़का खेलता है, लड़के को बुलाओ
The boy plays, call the boy
Usage Notes
The key rule: masculine -आ ending nouns change to -ए before any postposition. लड़का (boy, direct) → लड़के को (to the boy, oblique). Feminine nouns don't change in the singular.
Adjectives also change to match: अच्छा लड़का (good boy, direct) → अच्छे लड़के को (to the good boy, oblique).
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Related — Foundational Grammar
Hindi nouns and adjectives are either masculine or feminine, which affects verb endings, adjective forms, and postpositions
Hindi nouns, adjectives, and verbs change form for singular (one) vs plural (more than one)
The base form of a verb — remove -ना from the infinitive to get the stem used in all verb forms
Verb forms used as adjectives — habitual (V-ता हुआ = 'doing') or perfective (V-आ हुआ = 'done')
Comparative (-तर) and superlative (-तम) suffixes, or ज़्यादा (more) and सबसे (most) in everyday Hindi