Example 1
A cup of hot cocoa sounds delicious. I'm so cold that I can't feel my toes.
In retrospect, wearing sandals in January was probably a bad idea. You know what's a good idea? Using a period to separate two sentences like this.
Example 2
I'll meet you in front of the movie theater at 6:30 p.m.
Did you know that p.m. stands for post meridiem, which means after midday? Look at us, dropping clock knowledge. Here's some more info: if a sentence ends with an abbreviation, like it does here, the sentence still only has one period at the end.
Example 3
Ellie wondered if dragons were real.
Even though Ellie's questioning the existence of dragons in this example, it's still correct to use a period here because the sentence is stating an indirect question.