Quite literally, 快乐【kuàilè】is the clauses to the English word 'happy' in which快【kuài= fast, happy】and 乐【lè=happy】.
a. 快乐【kuàilè】is often used to wish people to be happy in special occasions and festivals.
Example 1:
Merry Christmas
圣诞【shèngdàn=Christmas】快乐【kuàilè=happy】!
圣诞 快乐【kuàilè=happy】!
Example 2
Happy Thanksgiving Day
感恩节【gǎn'ēnjié= Thanksgiving day】快乐【kuàilè】!
感恩节 快乐!
b. 快乐【kuàilè】can be used for personality of someone.
Example:
Mary is a very happy person.
玛丽【Mali=Mary】是【shì=is】一【yī=a】个【gè=a measure word for persons】很【hěn=very】快乐【kuàilè=happy】的【de】人【rén=person】。
玛丽 是 一 个 很 快乐的人。
c. 快乐【kuàilè】is used to express quite extended happy feeling.
Example:
Tina is very happy with her job.
Literally means," Tina loves her job, she is very happy".
蒂娜【dìnuó=Tina】喜欢【xǐhuān=love】她的【tā de=her】工作【gōngzuò=job】,她【tā=she】很【hěn=very】快乐【kuàilè=happy】。
蒂娜 喜欢 她的 工作,她的 很 快乐。
d. 快乐【kuàilè】can also be used to express quite extended happy feeling in a period of time.
Example:
I was very happy in elementary school.
Literary means, "When I was in elementary school, I was very happy".
小学【xiǎoxué=elementary school】的【de】时候【shíhòu=when】我【wǒ=I】很【hěn=very】快乐【kuàilè=happy】。
小学 的 时候 我 很 快乐。
e. 快乐【kuàilè】can means 'happiness'.
Example:
Don’t build your happiness on the pain of others.
别【bié=don’t】把【bǎ】自己【zìjǐ=oneself, own】的【de】快乐【kuàilè=happiness】建筑【jiànzhù=build】在【zài=at】别人【biérén=other people】的【de】痛苦【tòngkǔ=pain】上【shàng=up】。
别 把 自己 的 快乐 建筑 在 别人 的 痛苦 上。