"Multiply two by three, get six."
Translation:E hoʻonui i ʻelua ma ka ʻekolu, he ʻeono.
I don't understand when to use "i" and "ka" in these sentences and when to not use them.
It depends, both when giving order nor statement of a equation and not.
E hoʻonui i ʻelua ma ka ʻekolu, he ʻeono = Multiply two by three get six.
ʻElua hoʻonui i ka ʻekolu he ʻeono = Two times three is six.
Notice some difference or so in the two specific sentences?
A good way -if not the best- to remember this stuff is by writing them down and mark where the changes are. P.S lawe (minus) and puʻunaue (divided) are similar to each other.