For language geeks: https://kpt.datamediate.com

KPT is a language app specifically targeted at explainable verb conjugation for highly inflected/agglutinative languages. Currently works for Finnish, Ukrainian, Welsh, Turkish and Tamil.

These are really hard languages to learn for most speakers of European languages, particularly English - we're not used to complex verb conjugations, they're hard to memorise and the rules often feel quite arbitrary. Every other conjugation practice app just tells you right/wrong with no explanation, which doesn't really help you learn when there are literally hundreds of rules to get right.

The interesting part was using an LLM to create a complete machine-executable set of conjugation rules, which are optimized for human explainability, and an engine to diagnose which rule is at fault when you get it wrong. There's several hundred rules needed for each language in order to cover all exceptions.

NB as a bonus it also works fully offline because my best practice hours are when I'm travelling and have poor connectivity.