In an earlier stage of Greek, the genitive singular ending -σο was added to the stem ἀγρο, giving ἀγρόσο (the accent remaining on the same syllable as in the nominative - so I guess we'd have seen τόσο for the article, which at that time was a demonstrative I believe). Then the σ, as it was between two vowels, disappeared, and the vowels contracted: ο+ο = ου. As usual with contraction, the accent remained on the first part of the new vowel sound, so there's a circumflex - ἀγροῦ and τοῦ.
In the case of τούς, it's like ἀγρούς - here, the older ending was νς, so the forms were ἀγρόνς and τόνς. This time the ν disappeared, and compensatory lengthening took place - the ο lengthened into ου. Because this was lengthening rather than contraction, the accent stayed as an acute.
Statistics: Posted by MattK — Wed Oct 11, 2023 9:36 am