it's not quite the same. The statement 'usijaribu' is imperative. 'You do not try' would be '(wewe) hujaribu'. Note that 'hujaribu' means both 'you do not try' and 'he/she tries', the difference lies in intonation
I've found the answer in the mean time:
This is the negative form:
- Singular: u + si + stem
- Plural: m + si + stem
If the last letter in the stem is an a, it is replaced with e. (not applicable here)
So this is the singular form: u + si + jaribu.