KUAI-4

('kuai' in the fourth tone)

There are 17 standard Chinese characters with a 'kuai' pronunciation. In a full nine of these, 会 is the sound-indicating component. In another four, it's what is to the right of the 忄 in 快.

快 = quick, fast

请快! = Qing(3) kuai(4)! = Please hurry! (lit. Please quick!)

Or, more polite:

请快一点! = Qing(3) kuai(4) yi(1) dian(3)! = Please hurry a little!

While 急 is only used in negative commands or requests, 快 is only used in positive ones. In Chinese, it would sound strange to say '不用快'.

不用急! = Bu(4) yong(4) ji(2)! = No need to hurry!

However, to express the opposite (Please hurry!), 急 is not used.

急 = ji(2) = to hurry

***

快乐 = kuai(4)le(4) = happy

***

愉快 = yu(2)kuai(4) = pleasant

祝你旅途愉快! = Zhu(4) ni(3) lv(3)tu(2) yu(2)kuai(4)!  = Happy trip! (Lit. Wish you on a journey pleasant!)