I suggest you watch this video. It's only one minute and it explains a lot. Hope this helps!
https://www.khanacademy.org/math/ap-statistics/analyzing-categorical-ap/analyzing-one-categorical-va...
The heavy rain lashed against the window; (Independent clause).
For an independent clause, you would just add another sentence, such as:
The heavy rain lashed against the window; I watched it with a solemn expression.
But, here you can add any sentence you wish.
I think the one word would be "natural" (sorry if i misspelled)
-Salutation
In the format of informal letter yours affectionately' is termed as Salutation.