All the following information comes from Christopher Upward's The History of English Spelling:
Words that lost initial ‘h’:
OE hlaf ModE ‘loaf’
OE hlud ModE ‘loud’
OE hlædder ModE ‘ladder’
OE hlafdiȝe ModE ‘lady’
OE hliehhan ModE ‘to laugh’
OE hlid ModE ‘lid’
OE hnecca ModE ‘neck’
OE hnæȝan mod ‘neigh’
OE hreddan ModE ‘to rid’
OE hreoȝ ModE ‘rough’
OE hrycȝ ...
The form "třmi" is actually documented as the Old Czech form of the number
třmi = třemi
Zdroj: Šimek, F., Slovníček staré češtiny. Praha: Orbis, 1947.
Dalimil: "Na léto Tateři pojidú a třmi prameny vnidú."
Gebauer in his Historical Czech grammar, Part I, point 66 "trt, tr̥t from trьt" ...
One well-known example is the changes to vowel harmony in Korean, from the Middle Korean period (13th-15th centuries) to the modern day.
In Late Middle Korean, the yang/bright vowels were ㅏ /a/, ㆍ /ʌ/ and ㅗ /o/, while the yin/dark vowels were ㅓ /ə/, ㅜ /u/, and ㅡ /ɨ/. The vowel ㅣ /i/ was neutral. There is some debate on the precise realisations of these ...
"Cat" and "cats", "dog" and "dogs" are two separate words, but you can derive "cats" from "cat" by applying a rule. The same holds for languages with vowel harmony. If (like in Turkish) there is a suffix that changes depending on what it is added to, you just learn the roots and affixes, plus the ...