I have noticed lately that the question mark is having a difficult time asserting itself. In an email to a client recently, a student wrote I have collected the information you requested. Would you be able to meet this Thursday afternoon. His second sentence here is directly asking a simple question, so shouldn’t it conclude with a question mark? It should.
English grammar recognizes in the main two kinds of questions, direct and indirect. If I have something to ask you and I ask you with the reasonable expectation that you will answer as directly as I have asked, then I finish my statement with a question mark: Who called you so late last night? What time was it? Why? These are called direct questions. The statement is the question itself, and to mark that, a direct question requires a question mark.
If I pose my question, though, as part of a larger declarative statement, I can ask it indirectly: I asked you who called so late last night. In this construction, the sense of posing a question is carried by the meaning of the main subject and predicate, I asked, and the specific question I am asking is cast into another clause introduced by an interrogative pronoun, who. Indirect questions do not conclude with a question mark and, importantly, do not place a comma after the main verb (here the verb asked).
We should note, too, that some indirect questions can be a bit coy at times. There is nothing demurring or overmodest in the indirect question I asked you who called so late last night, but there is if I change the main verb: I wonder who called so late last night. The verb wonder carries its interrogative meaning more lightly. The indirect grammatical construction remains the same, but the question is posed more suggestively, opening up a whole new world of covert possibilities in the conversation. Indirect questions, then, can be both expressed (I asked you who) or implied (I wonder who).
One close friend of the indirect question is what the Chicago Manual of Style calls the courtesy question. If my direct question has failed to elicit a straight answer from you about who called last night, I might be impatient to cast an indirect question or not yet ready to blast a real imperative at you. Between these two guns, though, I have the courtesy question, which is really, as the Chicago Manual says, a request in disguise: Would you tell me who called last night. I do not expect you to reply “Yes” or “No,” so the courtesy is not a direct question. Instead, I am telling you to answer without telling you to answer. Because courtesy questions do not expect an answer, they do not conclude with a question mark.
But, of course, if all else fails, I can always escalate the conversation to hazardous new heights with the straight-up imperative: Tell me who called you so late last night! Like the emotionally fraught assumptions they frequently carry, imperatives often end badly—but more often than not, badly with an exclamation mark.