Answers:
My current pet-peeve is inconsistencies in English numbers.
Let's examine.
My 4-year old daughter is learning to count, and if our number system was consistent, she'd be able to count to infinity by now.
One, two, three, four, five, six, seven, eight, nine - No problems yet.
Let's skip "ten" for a minute. Eleven? What's that? Though it should, the word has nothing in common with "one", "twenty-one", "thirty-one", "forty-one". Maybe it should be "one-teen". Better yet, call it "onety-one".
Good: forty (except for the spelling), sixty, seventy, eighty, ninety
Bad: ten (should be onety), twenty (should be twoty), thirty (should be threety), fifty (should be fivety)
Thirteen should be "onety-three", Fifteen should be "onety-five" (or at least, fiveteen).
Imagine how easy it would be to learn (and teach four year olds) how to count beyond ten if all the numbers followed a consistent pattern.
one, two, three...
onety, onety-one, onety-two, onety-three...
twoty, twoty-one, twoty-two, twoty-three...
threety, threety-one, threety-two, threety-three...
fourty, fourty-one, fourty-two, fourty-three...
fivety, fivety-one, fivety-two, fivety-three...
sixty... Everything from 60 - 99 could stay as-is.
If Fred were around to submit a bug report when the guy who invented numbers was giving them names, we'd have a lot more free time, and a consistently named number system.
To answer your question
Trying to figure out why something is spelled the way it is in
English is often frustrating. :-) Spelling didn't become very
'stable' until well after the time of Shakespeare; if your 7th
and 8th graders haven't seen Shakespearean spelling yet, it might
be an eye-opener for them.
A better question to ask of a good etymological dictionary like
Webster's Second International might be where the words four and
forty come from. My dictionary says this:
ME. is Middle English, the language of England between about
1100 and 1500 A.D.
AS. is Anglo-Saxon, the language of the Saxon tribes that invaded
England in the 5th and 6th centuries - from about 600 A.D.
OS. is Old Saxon, the language of the original Saxon tribes of
northwest Germany between the Rhine and the Elbe rivers.
forty: ME. forti, fourti, fowerti, from AS. feowertig;
akin to OS. fiwartig, fiartig
four: ME. four, fower, feower, from AS. feower;
akin to OS. fiwar
So four and forty were different words starting a long time ago but
were spelled with the same beginning in Old Saxon and Anglo-Saxon
and part of Middle English. Somewhere along the way during the
Middle English era the simpler spelling of forty took hold and has continued ever since.
Of course it was easier to change spellings before there were
dictionaries and teachers paying close attention to your spelling.
:-)
It's interesting to notice the British spellings of words like
humour and labour, etc., which are spelled humor and labor in
the United States.