Adjective : The poor should be considered by the government. ( They should...)// The rich should pay higher taxes. The underlined words are functioning as nouns.
Infinitive: To smoke is dangerous. (It is dangerous.)
Gerund: Smoking is dangerous ( It is dangerous..) // I like reading books. (I like it.)
Noun clause: That he is a liar will be in the news. ( It will be...) // I do not think that he is married. ( I do not think so)
All the underlined elements can be subtituted for a pronoun or so : personal or impersnal. Look at the information in parentheses. So replaces a noun clause when the noun clause comes after a verb like "to think".
