Questions can be divided into yes-no questions (also known as polar questions) and wh- questions (also known as constituent questions), according to the expected answer. As the name implies, the answer to a yes-no question is either 'yes' or 'no.' The answer to a wh- question is expressed by a constituent that corresponds to the wh- phrase in the question. Wh- phrases are so called because they generally begin with wh- in English (who, what, which, where, when, why). Note that how counts as a wh- expression by virtue of its meaning, even though it doesn't begin with wh-. The term wh- phrase is standardly used even for languages other than English.
The distinction betwen yes-no questions and wh- questions is illustrated in (1) and (2).
(1) | Yes-no question | Has he called? | |||
(2) | a. | Wh- question | Who just came in? | The boy from next door. | |
b. | Who(m) did you invite? | All my friends. | |||
c. | When did she call? | After dinner. | |||
d. | Why did he do that? | Out of ignorance. | |||
e. | How did you fix it? | With the right tool. |
The questions in (1) and (2) are all direct questions. Indirect questions corresponding to them are given in (3) and (4). Here and in what follows, indirect questions are enclosed in square brackets.
(3) | Indirect yes-no question | I can't remember [ | ||
(4) | a. | Indirect wh- question | I can't remember [ who just came in. ] | |
b. | I can't remember [ who(m) you invited. ] | |||
c. | I can't remember [ when she called. ] | |||
d. | I can't remember [ why he did that. ] | |||
e. | I can't remember [ how you fixed it. ] |
(5) gives examples of various syntactic contexts in which indirect questions occur.
(5) | a. | Complement of adjective | I'm not sure [ whether they are coming. ] | |
b. | Complement of preposition | The question of [ whether they are coming ] remains unresolved. | ||
c. | Complement of verb | She asked [ whether they are coming. ] | ||
d. | Subject | [ Whether they are coming ] remains up in the air. |
Finally, indirect questions can be finite or nonfinite, as shown in (6).
(6) | a. | Indirect wh- question, | finite | They can't remember [ what they should pay attention to. ] | |
b. | nonfinite | They can't remember [ what to pay attention to. ] | |||
c. | Indirect yes-no question, | finite | They can't remember [ { whether, if } they should turn off the lights. ] | ||
b. | nonfinite | They can't remember [ { whether, if } to turn off the lights. ] |