A declarative sentence (also know as a statement) makes a statement and ends with a period. It's named appropriately because it declares or states something.
It might be more helpful, though, to think about what these kinds of sentences don't do. They don't ask questions, make commands, or make statements with emotion.
I like sleeping.
Yesterday, I slept through my alarm.
If I don't wake up when my alarm goes off tomorrow morning,
please scream until I stand up and you're sure that I'm awake.
Every sentence that you have read on this page so far fits this description.
This sentence doesn't! Can you tell why?
You knew why, right? It must end with a period, not an exclamation mark or a question mark.
Sentence diagrams are neat little things that show us how the words in sentences are related. Every single sentence must have at least a subject and a verb. Here is how you would diagram those two things.
Okay, just for fun, let's ramp things up and look at some more complicated examples.
Please don't get overwhelmed if you've never seen a sentence diagram before. Just sit back and take it all in! (You can learn step-by-step here if you'd like.) Here are three quotes from some old guys.
"A man can not be comfortable without his own approval."
- Mark Twain
"The thing about quotes from the Internet is that it is hard to verify their authenticity."
- Abraham Lincoln
In this next quote, you get two statements!
"A teacher affects eternity. He can never tell where his influence stops."
- Henry Brooks Adams