When learning Japanese, it’s important to understand how Yōon characters are pronounced. Yōon characters (拗音) are kana characters which end in an ‘i’, however have their pronunciation altered through the use of a ‘y’ or ‘j’ sound by adding a smaller や (ya) ゆ (yu) or よ (yo) character next to the kana character ending in ‘i’. For example, the normal kana character き (ki) could become either きゃ, きゅ, or きょ by adding a smaller ‘y’ character next to it. The pronunciation changes by replacing the ‘i’ sound of the character with the ‘ya’ ‘yu’ or ‘yo’ sound of the smaller ‘y’ character.
For example, きゃ would be pronounced ‘kya’ (notice we cut the ‘i’ sound of ‘ki’, only keeping the ‘k’, and add the ‘ya’ sound from the smaller ゃ (ya) kana. きゅ would be pronounced ‘kyu’ and きょ would be pronounced ‘kyo’ (also following the same rule).
The only types of characters that can become Yōon characters are kana that end in ‘i’, therefore only the following characters may become Yōon characters – き (ki), し (si/shi), ち (ti/chi), に (ni), ひ (hi), み (mi), り (ri). In addition, any dakuten/handakuten that also ends in ‘i’ can become a Yōon character.
Now there are some slight exceptions to the above rule. If you’ve learned the kana charts, then you’ll know that the characters し (si/shi) and ち (ti/chi) aren’t pronounced as si and ti, but rather as shi and chi. To pronounce their Yōon equivalents, once again simply drop the ‘i’ sound. Since we’re keeping the ‘sh’ and ch’ sounds, we’ll need to drop the proceeding ‘y’ sound from the ‘y’ kana. So for example in しゃ (sha), we drop the ‘i’ from ‘shi’ making ‘sh’, and we drop the ‘y’ from ‘ya’ making ‘a’. We combine ‘sh’ and ‘a’ together, making ‘sha’. This same rule applies to all the shi characters – しゅ (pronounced shu) and しょ (pronounced sho).
The same rule also applies to the ち (chi) character when converting it into a Yōon character. We keep the ‘ch’ sound, drop the ‘i’ sound, and drop the ‘y’ from the proceeding ‘y’ kana. So for example ちゃ is pronounced cha, ちゅ is pronounced chu and ちょ is pronounced cho.
In terms of the dakuten/handakuten characters, we can convert them into Yōon characters by following the same rules as set out in this section.
The easiest way to remember how Yōon characters change is to follow this simple rule: If an ‘i’ kana is pronounced with two letters, then drop the second letter only, and add the full ‘y’ kana sound. If an ‘i’ kana is pronounced with three letters, then drop the third letter only, and drop the ‘y’ sound from the ‘y’ kana.
Note: The rules covering Yōon characters apply both to the Hiragana and Katakana.