Having trouble differentiating long vowels and monophthongs in Finnish?

I have a huge problem when I try to pronounce Finnish words, for instance: 'aa' and 'ai'. I've looked at the IPA and it shows that 'aa' has a short 'i' after it, and 'ai' basically is as it's shown. But since i haer them both said the same how do I tell them apart? I would say a word for like for instance: kohtaan, I would say it as kohtain because I've seen the IPA show 'aa' as 'ai' but the 'i' is short and I end up saying 'ai' for some reason. Sometimes I would try to raise the tone of the vowel and say 'aa' like the 'a' in 'father' and I would normally say the vowel 'a' as in the word 'omatunto' as the 'a' in 'about'. Any help would be good, thanks.