それが決まったら知らせるよ。

that (indicating an item or person near the listener, the action of the listener, or something on their mind); it; that time; then; there (indicating a place near the listener); you

indicates sentence subject (occasionally object); indicates possessive (esp. in literary expressions); but; however; still; and

to be decided; to be settled; to look good in (clothes)

to notify; to advise; to inform

indicates certainty, emphasis, contempt, request, etc.; used when calling out to someone; used to catch one's breath or get someone's attention; yo!