C. Wh-words are what, when, where, who, which, why and how. We use clauses with a wh- word.
C1. We use who to ask questions about people.
C2. use whose to ask about possession.
C3. use what to ask questions about things.
C4. use which to ask someone to choose something.
C5. can also use what and which with nouns.
D1. after verbs of thinking: know, understand, suppose, remember, forget, wonder. We also use clauses with if.
D2. after verbs of saying: ask, say, admit, argue, reply, agree, mention, explain, suggest
D3. tell and some other verbs of saying must always have a direct object: tell, remind
D4. after some verbs of thinking and saying we use wh-words and the to-infinitive.
D5. When the subject of the to-infinitive is the same as the subject of the main verb.
D6. When the subject of the to-infinitive is the same as the person spoken to.
D7. after some nouns to say more about the noun.
D8. We often use a wh-clause after is.