Forty is missing the U that four and fourteen have, because the vowel sound changed in a longer word and the spelling followed it.
First, how do you pronounce it?
FOR-tee
The first part sounds exactly like the word for, and in most accents for and four sound the same. So the U in four has no sound of its own.
The weird part
Look at these number words:
four → has a U
fourteen → has a U
fourth → has a U
forty → no U!
They all come from the number 4. They all sound alike at the start. Only one of them drops the letter.
Other number words change in the -ty tens too: nine becomes ninety, three becomes thirty, and five becomes fifty. The tens often shorten or change their base word. But with forty, the sound stays the same and only the spelling changes.
What happened to the U?
Word historians do not all give the same answer, and English spelling history is messy. Older books show many spellings of the number.
One careful explanation says it is about how the vowel behaves. In older English, the vowel was long in both four and forty. In a two-part word like forty, the vowel was shortened. Later it changed again, but spelling had already moved on. Spelling followed the shorter version, and the U was dropped.
Four kept its ou spelling. The word fourteen should have been spelled like forty, but it was pulled back to the spelling of four. So the U stayed.
The reveal
Four and fourteen kept the U. Forty lost it. Today the sound is the same as in four, but the spelling kept the shortened old version.
⚠️ Common Mistakes
Do not put the U in forty.
❌ fourty
✅ forty
Do not drop the U in fourteen.
❌ forteen
✅ fourteen
Fourth keeps the U too.
❌ forth place (when you mean position)
✅ fourth place
💬 In a Conversation
Sam: How do you spell 40?
Mia: F-O-R-T-Y.
Sam: And 14?
Mia: F-O-U-R-T-E-E-N.
Sam: So where did the U go in forty?
Mia: It took a day off.
⏳ The OddLingo Timeline
long vowel in "four" and "forty" → shorter vowel in two-part "forty" → spelling follows: ou becomes o → "four" and "fourteen" keep ou
🥡 The OddLingo Takeaway
Forty is for + ty. Four, fourteen and fourth keep the U.
If you can say FOR at the start of forty, you are halfway to spelling it. Just leave the U out.