I was taught the difference between pole and samahani is that pole means 'I feel sorry for you' and samahani means 'I'm sorry', as in sorry for something you have done or said
It also helped me to remember samahani as "Excuse me." (in most contexts it can be translated that way correctly)
Pole, kaka, nimeshaanza kula oti.
(P.S. Not sure if it's correct, so it needs proofreading.)
The grammar is correct, the content confuses me more "Sorry, brother, I have already begun to eat the oats." :D