What is the proper grammar of giving/receiving mishloach manos/shalach manos? I've heard people use both terms as in, "I'm preparing mishloach manos for tomorrow" and "I hope to bring the shalach manos later".
THe proper form is "Mishloach Manot/s" - משלוח מנות as it appears in the Meggilah:
"לעשות אותם ימי משתה ושמחה ומשלוח מנות איש לרעהו ומתנות לאביונים"
Compare to לנוח - מנוח, or לקחת - מלקוח etc.
One can use derivatives of "send" - לשלוח, like "אני שולח מנות", "שלחתי מנות" etc.
There's no such form of a noun as שלח מנות in Hebrew.
As Al Berko said, the version in the Megillah is mishloach, in binyan kal.
In Mishnaic Hebrew, the word for sending things (and even sending מנות!) is written in binyan pi'el. As far as I can tell it means the same as binyan kal in this context*.
בֵּית שַׁמַּאי אוֹמְרִים, אֵין מְשַׁלְּחִין בְּיוֹם טוֹב אֶלָּא מָנוֹת
My guess is שַׁלַּח comes from the pi'el form of the verb, as in ויהי בשלח פרעה.
* I don't see how this distinction can be applied here since you're sending stuff, not people