Translation:The bananas have not been cooked
Ndizi -- hazi--ja--pikwa
banana--they(neg.)--have not--been cooked.
It will be slightly easier once you've hit object infixes, provided you take note of the sentence saying that infixes for noun classes other than M/Wa are the same as their subject prefix (don't worry, it lists them). When you reach it I suggest you copy them down somewhere.
Negative subject prefixes are handled the usual way.
So for this sentence example:
Ndizi is an N/N noun. It's subject prefixes are i- for singular and zi- for plural. The negative forms are hai- and hazi- respectively. Since this sentence uses hazi-, we know there are multiple bananas.
The -wa at the end indicates the passive voice (again, will make more sense when you reach the relevant skill). So where pika would mean the banana has cooked, pikwa means the banana has been cooked.
So far we have worked out that there are multiple bananas, and that cooking has not been happening. (hazi-pikwa).
The reason I have left -ja- to last is because it's an error of omission in the Immediate Past tips and notes. I've just sent a message to Branden on his activity stream, hoping he'll see it and add some detail. (report date: 14 Mar 2017)
It's actually fairly simple: -ja- is the negative immediate past tense. Just like -ku- is the negative of the past tense -li-. So really all you have to do is put -ja- instead of -me- if the immediate past is negative, and drop the infinitive ku for monosyllabic verbs (nimekula/sijala. I have eaten/I have not eaten).
Disclaimer: I started this course the day it hit limited beta, with just a tiny bit of understanding of the language. I did a lot of zipping round various parts of the tree to find this info (plus some googling once I figured out what the missing bit was). While I have learnt a lot, I have not suddenly become an expert during these few weeks. So if at any point you find a better explanation, use that over what I say here. :)