"Embe lisipoiva sitalila"
Translation:If the mango does not ripen I will not eat it
Agree! However, if you start from the back until you find something you know (here: 'iva') you can do some research, and it is possible to get behind the magic. Good place to start would be here: https://en.wiktionary.org/wiki/iva#Verb_2 Click on "conjugations of -iva" and then on "If Not" ... Really a good source to look for anything, even conjugations. If you are interested in more, do the same with the next verb 'kula'. Start from the back until something familiar appears ('la'), then look here https://en.wiktionary.org/wiki/la#Swahili and click on "Conjugations of -la" ... But I agree, should be explained in the course, too.
siiva does not exist. -si- is only used with certain constructions, not on its own. -sipo- is the rough negative equivalent of -ki-. If you would use kama and the future tense if this were positive, the negative equivalent (which I think is also correct but I'm not sure) would be kama embe halitaiva.
Also, -bivu should be bivu, not mbivu in your second example. Embe is a class 5/6 (Ji-Ma class) noun, so there's no prefix in the singular.
And furthermore, the meaning is not the same.
(kama) embe lisipoiva = if the mango does not ripen
kama embe si bivu = if the mango is not ripe
The verbs -iva and "ripen" indicate the transformation from unripe to ripe. The adjectives -bivu and "ripe" indicate the resulting state that occurs after this transformation. It may seem like picky details, but it's a different meaning. Using -bivu like this could mean that the mango has already ripened and is currently ripe. Using -iva means we know the mango is not ripe now.