“All that sun must have really worn them out.”
You're riding home after a day at the beach with your young children. The children have fallen asleep in the backseat. You tell your husband this.
All that sun must have really worn them out.
"The sun" is what we call the big ball of fire up in the sky. It's almost always "the sun", not "a sun". There's only one of them, and everyone knows which sun you're talking about, so we use "the" before "sun".
But you may also hear people using "sun" in other ways. For example:
This means that the listener's skin is tanned or maybe burned a little bit.
In this example, "all that sun" means "all that sunlight that they were exposed to".
I could use a little sun.
When people use "sun" in this way, they're really talking about the light from the sun, or the effects of the sun on a person's skin.
You can make guesses about things that happened in the past with the phrase "must have ___":
A: Where are Deanna and Jun?
B: They must have left already.
People use "must have" when they have a pretty strong reason or evidence for thinking something. You don't use "must have" when you're just making a guess about something you don't know about.
Use the phrase "___ wore me out" to mean that something made you tired. Here are some examples:
When you're at the top and everyone's out to get you, it wears you out mentally.
All the fighting and arguing just wore me out. I had to go in the other room to be alone for a few minutes.
From these examples, you can see that you can be worn out "mentally" (in your mind). You can also get worn out physically (in your body):
You're going to wear yourself out really quickly trying to carry that much.
And you can also use the phrase "wear ___ out" to talk about physical objects as well as people. To "wear out" an object means to use it so much that it start to look old and maybe starts to break:
It only takes me about six months to totally wear out a pair of sneakers.