1. The weather is not what it was the day before.
2. That is why she has come to us.
3. The problem is that he has lost her telephone number.
4. The question is whether they will be able to visit us.
5. The question was why no one had helped the old woman.
Придаточные предложения соеденяются с главным такими союзами как - that, wether, if.
Или союзныи словами - Who (whom), whose, what, which, when, where, how, why.