ha

Ha is the noise of the penny dropping: recognition, sudden understanding, mild surprise. Ho is the other one and it leans towards a sigh or a direct address, ho, mia amiko. Zamenhof kept both and the corpus keeps them apart fairly consistently, so the split is worth learning rather than treating them as free variants. Doubled up, ha, ha is written laughter. Like all interjections it takes no endings and sits outside the grammar entirely.