where is the 'not' here
The "not" as you put it is in the prefix "Hu" and the changing of the ending vowel to "i". This applies to all verbs in Kiswahili.
Pika -- Hupiki
Cheka -- Hucheki
Piga -- Hupigi
Not quite all verbs. There are foreign verbs, mostly from Arabic, which don't end in a, and their ending doesn't change. Eg. kujaribu > hujaribu
How come sometimes it is 'he does not ...' and other times it is 'you do not..'? I dont understand how you can determine the difference with 'hu' ?
Hu is always you. He or she does not is 'ha'