221. The preposition σύν (or ξύν) means in company with. It is not used as a pure adverb, but is found in tmesis.
Il. 1.579 σὺν δʼ ἡμῖν δαῖτα ταράξῃ
and disturb (συνταράσσω) our feast
It is used with an instrumental dative (§ 144).
To express equally with, or at the same time as, Homer uses ἅμα with a dative; while σύν commonly means attended by, with the help of, etc.
with armor on
aided by Athene
So Il. 4.161 σύν τε μεγάλῳ ἀπέτισαν they pay with a great price.
The use of σύν with the dative has been recently shown by Tycho Mommsen to be confined, generally speaking, to poetry. The Attic prose writers (with the singular exception of Xenophon) use μετά with the genitive; the practice of the poets varies, from Homer, who hardly ever uses μετά vwith the genitive, down to Euripides, who uses it about half as often as σύν. It is evident that in post-Homeric times μετά with the genitive became established in the ordinary colloquial language, while σύν with the dative was retained as a piece of poetical style, but gradually gave way to living usage. See Tychο Mommsenʼs dissertation Μετά, σύν υnd ἅμα bei den Epikern (Frankfurt am Main, 1874).