“You're the one that kept complaining about how old and ugly our TV was!”
You bought a new TV for your wife for her birthday, and she's not happy. She says she didn't want you to buy it. You bought it because she was complaining about the old TV, so you can't understand why she's upset. You blame her, and say this.
You're the one that kept complaining about how old and ugly our TV was!
Use this kind of phrase when you want to emphasize the person who did something. In the situation above, if you said:
You kept complaining about how old and ugly our TV was!
...then the focus of the sentence is on "how old and ugly..." This sentence is just an explanation for why you bought the TV.
But when you use "you're the one who...", it focuses the sentence on the other person. One use of this phrase is to put the blame for something on that person. When someone blames you for a problem, but you want to say it was their fault, use this phrase:
A: This place sucks!
B: You're the one who wanted to come here so badly!
To "keep" doing something means to continue to do it, or to do it again and again:
Why does he keep calling me? I've told him I have absolutely no interest in him.
When you want to talk about the level or amount of a quality, you use the phrase "how ___ something is". For example, if it's been raining a lot and the ground is really wet, you can say:
I can't believe how wet it is.
Another example is when you meet someone who's famous and seems unfriendly, but he is actually a nice guy when you meet him: