When did he inform about his sin?
Rashi: He did so constantly; future tense used in place of present. When I told Nasan ha'Navi "Chatasi", You bore my sin - "Gam Hashem He'evir Chatascha
Is there any need to inform Hashem?!
Radak: The verse speaks in Leshon Bnei Adam (the way people speak); it means 'I will confess in front of You and not cover it up.'
Sha'arei Teshuvah (1:18): Even though a Ba'al Teshuvah should not publicize his sins, like we may infer from "uv'Kum Resha'im Yechupas Adam" (Mishlei 28:12), he must confess them.
What is the meaning of "Amarti"?
Radak: I thought [that it is good for me to confess my sin].
What is the meaning of "Nasasa"?
Radak #1: You will bear. Past tense used in place of future; this is common in Nevu'ah (Radak Yirmeyah 46:23 - for Nevu'ah for the future exists, as if it already came).
Radak #2: It is a Nevu'ah (that Hashem already bore his sin).
Malbim: Right after I resolved to inform my sin and confess, You immediately bore my sin and rain descended.
Why does it say "Avon Chatasi"?
Radak: The Semichus shows that it was great, like "Admas Afar" (Daniel 12:2), (is Samuch, even though they are the same) and "mi'Tit ha'Yaven" (below, 40:3).
