Different pronunciation with different "xあ+い" combinations?

[JacobARF]

Hi!

I've noticed that the い in 世界 (せかい) and はい are pronounced differently? In 世界 it's like "se ka i" but in はい it's like one quick word, "hai", almost a diphthong? Same for やさい and はやい. Is there a rule for this, or do you just learn word by word?

[richvh]

It may depend on where the tonal accent falls.