Is there any difference between these two questions? Is the second one grammatically right?

How do I know?


How am I to know?

  • 2
    They're both grammatical and correct, but mean (somewhat) different things. The question "How am I to know?" is the same as "How should I know?", that is: Why do you expect me to know? What reasons can you give me for your apparent expectation that I know that?. The first expression "How do I know?" can and has been used informally to express that same idea, but more formally is closer to an epistemological musing: How is that I came to know this thing?. So, in their formal senses, in the second question, you're implying don't know something, whereas in the first, that you do. – Dan Bron Mar 4 '16 at 2:31
  • @DanBron: I think that would do well as an answer with perhaps a bit more polishing. – Nathan Tuggy Mar 4 '16 at 2:34
  • @NathanTuggy The issue I have with posting an answer proper on ELL is that I am not versed enough in the mechanics of English (or linguistics in general) to justify an answer. And my instinct is that the implicit assertion "Trust me, I'm a native speaker" is insufficient and unfair to your community members who are learning English from the outside. Unless you regulars feel differently? – Dan Bron Mar 4 '16 at 2:36
  • @DanBron: Hmm, fair enough. I think you're pretty close to having enough justification, but I can see that there's a bit more that would be nice to have too. Perhaps I'll write up my take on why that is, if you don't mind giving me the inspiration. – Nathan Tuggy Mar 4 '16 at 2:42
  • @NathanTuggy I would love to see what you put together. I honestly learn more about English from you all on ELL than I do on ELU. – Dan Bron Mar 4 '16 at 2:43

Dan Bron's opening comment is correct, but I'd like to explain a bit more about why that's the case.

Basically, it comes down to tense and modality, or conditionality. Using "to be" (or in this case, "am") with an infinitive like "to know" is one of the future tenses English uses*, and it's a tense that expresses intention and plan. So you could rephrase "How am I to know?" as "What plan would you expect me to use to know this?"

Since that's a fairly common sort of thing to say, but "How am I to …" is a fairly formal pattern, it's not surprising that "How do I …?" has been adapted to that use as well informally. Formally, though, "do" is not conditional at all here. It's just asking for a explanation that matches current reality. So you could rephrase it as "Right here, right now, how is it that I know this?"

Obviously, both sentences (and both verbs) are very flexible, so there's a lot of variations possible, and a lot of different subtle implications depending on context. But that's the basic breakdown.

*To the extent that English has future tense, at least, which is the subject of some scholarly debate. Close enough, though.

  • Oh, interesting explanation. I'm going to have to mull it a bit. – Dan Bron Mar 4 '16 at 7:09
  • Understood. In addition, I have often read of such sentences, like those from arcticle titles: "Brazil to move...", "US to ban..." and alike. Are those a short form of that future tense? Does it mean "Brazil is going to move..."? – Gnoulv Mar 4 '16 at 12:46
  • @Gnoulv: Yep. "Headlinese" tends to leave out a lot of connecting pieces wherever possible, so it has some unusual appearances, but that's basically what that comes from. – Nathan Tuggy Mar 4 '16 at 17:56

Let's say you are solving a math problem You are stuck somewhere for hours and after hours of struggle you came to know the reason, that is you were unaware of a formula which happens to be out of scope for you. At that point you can say "how am i to know?" According to me, "How do I know" is more like a statement Let's say after hours of struggle you went for a drink and after returning you started again and you solved it. (rare scenario) Next day you find the solved problem in your bed. You checked and find a strange formula which solved it. At that point you may say "how do I know" since you don't remember solving it.

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