"Good evening, my friends!"
Translation:Добрий вечір, мої друзі!
It seems that Ukrainian has a lot of different letters for the "i" sound. Are there any rules that help with knowing which one to use, or do you just have to learn how to spell every individual word, like in English ?
И/и and І/і are different sounds. И is a mid-high vowel, while І is a high vowel.
И is comparable to the English i in 'ship', and І is comparable to the English ee in 'sheep'. However, in English the difference is not only in the vowel quality but also in the length (in quantity). In Ukrainian, the difference is in the quality only, not in the length.
Also, in standard Ukrainian і softens the preceding consonant (ліс 'forest' is pronounced with a soft ль), while и doesn't (лис '[male] fox' is pronounced with a hard л).
Ї signifies a combination of й+і, i.e. it's a consonant 'y' followed by a vowel 'ee'. It's not normally used after consonants nowadays.
(In the past, in some Ukrainian dialects, і didn't automatically soften the preceding consonant. In these dialects, ї was used for і after soft consonants: therefore, people wrote віз 'cart' with a hard в, but вїз 'drove' with a soft вь. This spelling is not used nowadays, we write both words as віз. In standard Ukrainian, both are pronounced with a soft вь.)