The hint for the mnemomic difference is the Kyoto きょうと … but they sound the same?
learning to tell them apart by sound is a little tricky, since English (for instance) for the most part doesn’t distinguish vowels by timing! they are in fact different, but not by a ton. when you’re next doing some listening practice, try just listening for the mora timings of words and see if you can start to notice things!
きょう and きょ are different in how long you pronounce the vowel sound (in this case the おsound). It can be difficult when you first start out to differentiate between the sounds because we don’t often think about how long we say vowel sounds in English. It’s sometimes helpful to imagine you’re actually saying each individual kana sound. きょ vs. きょ・う
A set of words that helped me out with timing was おばあさん (grandmother) and おばさん (aunt)/おじいさん (grandfather) and おじさん (uncle). If you listen to the pronunciations of these sets of words, you can see how much faster they say the ば and じ sounds.
In this video, you can hear the difference in length the most on the はと vs ハート.
On the visual/story mnemonic level, I differentiate きょう and きょ as:
きょう: Kyoto as it is.
きょ: ’Little Kyoto’, a miniature version of the city as it might exist on like a table in a museum, but filled with tiny people going about their tiny lives.
It’s super silly, but this distinction helps me remember which reading a kanji uses in my mnemonic stories. Like for 巨人 (きょじん - giant), I picture a giant stomping around Little Kyoto. Somehow it works…