The monks who devised the first English spelling system in the 7th century had a bit of a problem. – The biblical Latin on which they based it, used the letter v for both the /v/ and the short /u/ sound. This usage continued in English too until the 17th century. The Victorians still used it even in the 19th century, when they wanted to make inscriptions look older, as when restoring Chester Cathedral, in the words ‘vp’ and ‘vnder’.
Moreover, Latin has no sound like the English /w/. So the monks spelt it ‘vv’, i.e. with 2 v. That is why the letter ‘w’ is still called ‘double u’.
The printing press which Caxton brought over to in London in 1476, had the letters ‘v’ and ‘u’. But instead of using them phonetically, he opted for ‘v’ at the start of words (vnderdood, vsed, varyeth) and ‘u’ within words (neuer, haue, ouer).
The use of ‘v’ for both the /v/ and /u/ sounds, made words like ‘vvvnder’ and ‘lvv’ a bit tricky to read in handwritten manuscripts. So the monks decided to solve the problem by using ‘o’ instead of ‘v’ for the short /u/ sound, whenever /u/ occurred next to vv or v, e.g. wonder, lov.
They also thought that having any succession of several short strokes next to each other, as in ‘munk, munth, munger’ didn't look great. So they nearly always spelt short /u/ next to ‘m’ and n’ with ‘o’ as well (monk, month, monger).
This habit has become known among linguists as ‘minim stroke avoidance’ and is the reason why, 1500 years later, we still spell 53 common words un-phonically, with an ‘o’ for a short /u/ sound, instead of ‘u’.
Changing u to o after w, led to problems with words which had a short /o/ sound after ‘w’, as in ‘wos, wont, wosh’. The monks tackled this with another silly change. They spelt /o/ next to w with ‘a’: was, want, wash’’. This then led to decoding difficulties like ‘swan swam’.
Printing helped to sink English spelling even further into the mire. This was partly because Caxton’s own grasp of it was uncertain, after living on the Continent for 30 years, including some time in Cologne, where he learned to print.
Paying type-setters by the line was another reason. – To earn more money, they became fond of adding extra letters, especially an extra –e. This has left us with the likes of ‘love, glove, dove’.
The doubling of consonants was another favourite: ful, wel → full, well. Often they used both: had – hadde, then – thenne. Most of them were dropped later, but many have survived: come, some, done.
These, relatively minor changes of early scribes and printers continue to undermine sensible spellings, like ‘fun, run, bun’ and ‘bone, stone, alone’. Many of them continue to make learning to read harder too.