Translation:I am not eating
Kula is the infinitive. In the negative conjugation, the ku- is dropped from monosyllabic verbs (as opposed to the affirmative where it is kept). Also, in the negative, the final -a becomes -i. So we get ninakula 'I eat' but sili 'I don't eat'
Is there a way to tell when "Si" at the start of a word means "I do not" and when it means "I"
It's always negative "I + not". You might be confused by Sijambo, but that's essentially "I do not thing" ... it's just a weird phrase.