You gave us in lesson 14 “Zhu Ni Sheng Ri Kuai Le” for “wishing you a happy birthday” and in lesson 30 Wang says “Zhu Ni Ding Hun Yu Kuai” which you explained as “wish you a pleasant engagement”.
Are the expressions “Kuai Le” and “Yu Kuai” inter-changeable or would you use them differently, such as is one more casual than the other or is one more intense than the other?
Thank you for questions; I’ll do my best to answer them clearly.
KuàiLè and YúKuài both mean happy, but there’s a slight distinction between the two,
You’ve already guessed correctly that one is ‘heavier’ than the other:
KuàiLè expresses surface/temporary happiness, so it could be interpreted as joyous, cheerful, even laughing,
ShēngRì KuàiLè means happy birthday, and this ‘happy’ here is about having fun and enjoying yourself.
Similarly XīnNiánKuàiLè means Happy New Year.
YúKuài describes a state of mind; happiness
in the example Zhù Nǐ DìngHūn YúKuài you’re not just saying enjoy yourselves, have a laugh; no, you’re expressing wishes that the couple has a happy marriage and life together.
Hope that makes things clearer, and let me know if there’s anything else I can help you with.