Two of our "easiest" words: "to" and "of". Why their spellings?
I′m not sure I understand your question, but here′s a go at an answer anyhow:
The spellings of of and to are not at all easy in several respects:
The <f> spelling of [v] was fairly common in earlier English, but today the only remaining instance is of and its compounds like hereof, thereof, whereof. The history of <of>‘s spelling is not easily summarized and quite frankly not too helpful in explaining things, but here is a try: In earlier English a distinction was often (though not always consistently) made between the so-called “weak” and “strong” forms of a word. Our modern off started out as the strong form of of. Off came regularly to end with <ff> pronounced [f], while its weak partner has retained its early single <f> spelling and [v] pronunciation right up to today.
The fact that in of the <o> spells the short <u> sound is is due to vowel sound changes that occurred in early English — as in glove, brother, other, some and several other words.
The spelling of to is not quite so complicated: The <o> spelling of long <u> is quite rare, occurring in about 15 modern words such as approve, lose, move, tomb, whom, womb, and in who, two, do and to. This spelling has various causes. The words do and to earlier had a long <o> sound. But in the Great Vowel Shift of the 14th through 16th centuries, English long vowels systematically changed their pronunciation, with long <o> regularly becoming long <u>, as in do and to. No one knows for sure why the Great Vowel Shift occurred, but occur it did, and it has had some huge effects on English pronunciation and spelling. (Wikipedia has quite a good article on it.)
I apologize for the delay in this answer: I just discovered that we have a problem in having the website forward questions to my home computer. My email@example.com identity has apparently disappeared. I can be reached at firstname.lastname@example.org.