Hi Scott, that’s a excellent question. 了 is a very important word in Chinese. We can use it to express something happened/ happening/going to happen soon. Let’s look at these three sentences: 1.昨天下雨了！ 2.下雨了！ 3. 要下雨了！ 1. It rained yesterday! 2. It's raining! 3. it's going to rain! So when we say 刮风了！It means “The wind is blowing!”. 谢谢了！/ 再见了！，The tone is a bit stronger than without the “了”. 怎么了？*What happened?* “了” is for something already happened.

Yes, you are right! "了"used after the verb or adj. to indicate completion，also can use to : 1.indicates future certainty eg.下雨了！ 2.indicates the speeding up or stopping eg.别吵了！ 3.indicates interj. eg.太美了！

Correct! In most of the listed sentences, it has no meaning, just a symbol of past tense.

You are right,the了is not too much meaning in these sentence.

了indicates a change, or something has happened. For past time.