"Esther has eaten food"
Translation:Esther amekula chakula
Basically, it depends on what can take stress. Because the second last syllable is stressed and there are certain prefixes that cannot be stressed, this little "ku" is added to take the stress and prevent that.
The tense markers -na-, -li-, -me- and -ta- cannot be stressed, so if they are going to be the second last syllable of a word, you add "-ku-"
Ninakula, nilikula, nimekula, nitakula.
All of the subject and object markers can take stress, so that means, if you have an object, you don't use the "-ku-" ... Ninaila (I eat it). Also, the "ku" drops in the negative present because the subject can be stressed: sili.
The negative tense markers "-ku-" and "-ja-" can be stressed (although some people use them with the additional "ku"). So if you compare the positive and negative forms:
ninakula > sili
nilikula > siku(ku)la
nimekula > sija(ku)la
nitakula > sitakula
Some other verbal markers such as "-ki-" can be stressed:
But its negative equivalent "-sipo-" cannot because the second part, the "po", is actually a relative pronoun/affix and these cannot be stressed, so the negative version of above is:
In the subjunctive, the "ku" will not appear because the second last syllable will be a subject or object prefix (which can be stressed).
I hope I have clarified and haven't just further confused you.
You're welcome! The trick is remembering which verb forms have the -ku- and which don't.
Of the tense markers that can take the stress, I think it's only -ki-, -ka- and -a- (which is not taught in this course) among the positive ones and -ja- (neg. perfect) and -ku- (neg. past) among the negative ones.
You might find this helpful http://www.kwangu.com/swahili/verb_builder.htm