@user25750 `How many words should I use for politeness?` - Like many languages, more polite language tends to take more time... but that's a general observation, not a rule you can work from
kara na are two particles. kara can explain cause/motivation. na is a sentence-final particle that has a pretty good explanation here: https://japanese.stackexchange.com/questions/33969
-sai is at least referenced here:
https://japanese.stackexchange.com/questions/48566