My draft translation. Flowers, sceneries here are metaphors for people. From a oldie movie. Can be quite flirtatious because of the double meanings. Most translations of this song are not professionally done and many details are skimmed. https://www.youtube.com/watch?v=m0gCPAUDC8s -- very impressed you got the original lyrics! https://en.wikipedia.org/wiki/When_Will_You_Return%3F https://www.youtube.com/watch?v=ehokUX9U-DE -- Tango version from Alfred Haus 好花不常開Rare flowers do not bloom frequently -- denoting the girl 好景不常在Great views do not last -- denoting the guy 愁堆解笑眉Melancholy piling up, arch your eyebrows a make a smile 淚灑相思帶Tears soaked in a belt of yearning 今宵離別後After tonight's departure 何日君再來When will my lord be back -- An old term for the male lover 喝完了這杯When this cup's done, 請進點小菜Who will enter to order more food? -- the girl is a sort of a server in an old style restaurant inn 人生難得幾回醉In life, nary much to be in stupor 不歡更何待, Why wait? Be happy now -- literally 來來來Come, come, come -- the male customer 喝完了這杯When you finish this cup, 再說吧 let's chat then! 今宵離別後Tonight, after you left, 何日君再來When will my lord return? 逍樂時中有When in midst of leisure pleasure 春宵飄吾裁The spring wind waftered and I planted. -- Use your imagination. 寒鴉依樹尖The crow leaning against the bough tips in cold. 明月照高台The bright moon shines over the tall terrace 來來來 再敬你一杯 Let me toast you a cup -- from the male customer 玉漏頻相催The dripping rate of the jade clepsydra urges both of us -- use your imagination! 良辰去不回The good times will soon go and never to return 一刻千金價One moment is worth a thousand of gold pieces. 痛飲莫徘徊Drink away and hesitate not （來來來再敬你一杯）Let me toast you another cup 停唱陽關疊When the singing stops, the Yang Pass is piling high -- 陽關三疊 is a name of an ancient tune. It alludes a famous poem about parting of a friend. 重擎白玉杯Once more the jade cups held 殷勤頻致語In earnest, words exchanged hurriedly. 牢牢撫君懷Firmly and safely held by my Lord, （嘿Hey! 最後一杯One last cup, 乾了吧, Bottoms up!）

this is a very conservative way to express I really cherish the moment that you are by my side. However, good things don't always happen just like beautiful flowers don't always blossom. And the topic of the lyrics is after saying goodbye to you tonight, I wonder when will you visit me again?

That's ancient Chinese. It's pretty hard even for native speakers.

No morning sun lasts a whole-day. 好花不常開; 好景不常在

Actually there are translations on the internet :) It's really a beautiful song and lyrics. by saying "flowers and beautiful scenes are not lasting long..." is metaphorical approach to express she missed someone whom was her lover. you can check the translation of this lyrics by following link: https://www.youtube.com/watch?v=wYWEs6vVW6Q