"I am not going"
si- = "I" + "not" (a negative subject prefix - all the others start with "h", but there is si- instead of *hani-)
-end- = "go" (the verb stem)
-i = "not" (the marker of the negative present tense ... only used in verbs that end with "a", which all native Bantu verbs do)
Further to what Ben said, negative verbs in the present do not take a tense marker between the prefix and the verb stem. All other tenses do, so even if your verb isn't Bantu origin, you can work out the tense. Sijibu (I'm not replying) vs sitajibu (I will not reply), sijajibu (I have not (yet) replied) or sikujibu (I didn't reply).
So tense comparisons for negative kuenda:
Siendi -- I am not going.
Sikuenda -- I did not go.
Sijaenda -- I have not gone.
Sitaenda -- I will not go.