OA, OfA zy; MA dy, d; later d but also dy in various positions; Sam. often אד ; in Mand. written with a unique ligature developed from "d" + "y" and in our code transcribed as "d" with underscore. In some JBAmb also spelled as presumably pronounced: ת .

For a complete list of the examples introducing direct speech in the BT see J. N. Epstein, "Notes on Post-Talmudic Aramaic Lexicography", JQR 12(1922): 305.
All meanings of this particle, including the prepositional (see dy prep.) derive from meaning 2, fundamental to Semitic.
For the general tendency within Aramaic to expand the use ofdy/d--clauses for adverbial subordination see most recently N. Pat-El, Studies in the Historical Syntax of Aramaic (Gorgias, 2012), ch. 2.
The otherwise common usage introducing object clauses is rare in targumic texts, where Biblical Hebrew כִּי is regularly rendered by ארי or ארום instead.