"ku" can stand for "kun or kuin or koska" in spoken Finnish. The sentence decides its meaning.
Mä myöhästyin, ku bussi ei tullu. - I was late because the bus didn't come.
ku here: because / when - spoken 'koska'/'kun'
Written Finnish: Minä myöhästyin, koska bussi ei tullut.
No ku mul oli niin kiire. - Well, 'cause I was so busy.
ku here: because - spoken 'kun'
Written Finnish: No kun minulla oli niin kiire.
Mä jäin himaan, ku mua väsytti. - I stayed home because I was tired.
ku here: because - spoken 'kun'
Written Finnish: Minä jäin kotiin, koska minua väsytti.
Mä otin taksin, ku busseja ei enää menny. - I took a taxi because there were no more buses.
ku here: because - spoken 'kun'
Written Finnish: Minä otin taksin, koska busseja ei enää mennyt.
Tää on parempi ku toi. - This is better than that.
ku here: than - spoken 'kuin'
Written Finnish: Tämä on parempi kuin tuo.
Kesä on kivempi ku talvi. - Summer is nicer than winter.
ku here: than - spoken 'kuin'
Written Finnish: Kesä on kivempi kuin talvi.
Spoken and written Finnish are different registers. The examples show how "ku" is used here and pair it with the written sentence. Learn the meaning in context rather than replacing every occurrence of a word.