Ni and De vs Niwa and Dewa

What is the difference between Ni and Niwa? Same thing for De and Dewa.

I heard that Ni is used when you aren't comparing with others and Niwa is used when you compare, but I can't find any other sources to support it. So is it true, I'm confused.

And also when do you use Ni and when to use Niwa? Similarly for De and Dewa, what situation calls for which?

Yoroshiku Onegaishimasu.

Suppose you try to reach your friend Taro and (old-fashioned way) calls his home phone. His family answers.
- Taro-san wa imasu ka.
- Iie, ima wa imasen.
- Nan-ji ni kaerimasu ka.
- San-ji niwa kaerimasu.

- Is Taro in?
- No, he is not here now.
- What time does he return?
- By three o'clock he will return.

You see that in both cases "ima wa imasen" and "san-ji niwa kaerimasu," the "wa" is highlighting the fact "not NOW" and "by 3 o'clock (and not earlier)." In contrast, when you ask "at what time will he return?" you simply say "nan-ji ni."
"Wa" has the contrasting or highlighting function. Getting the hang of the function of this "wa" might take a bit of time.