Tara, our head writer, recently asked me what she should use as a heading in her blog post on vocabulary tips: Do you know? or Did you know? I automatically responded that either is correct, but then I started thinking more about this common ELT heading. Are both correct in all situations? What's the difference, if any?
They both make sense when asking about previous knowledge, grammatically speaking. Let's use the tip from Lesson 1 of our new Detective Series 2: False Alarm as an example. This tip describes what a false alarm is. If we use the heading did you know then we're asking "Did you already learn what a false alarm is at some point in the past?" If we use do you know then we're asking "Do you know, at this present moment, what a false alarm is?"
Although both make sense, we went with did you know because it sounds slightly more natural and common to our ears. Specifically, we used "Did You Know?" as the heading for our vocabulary and grammar tip box in our Detective Series 1 lessons. In our Detective Series 2 lessons, we updated that to the slightly more modern "Did you know…".
What about "Do you know these words?" With a full sentence like this, I think do you know sounds a lot more natural than did you know. I've never seen "Did you know these words?" in any textbook, but "Do you know these words?" is fairly common.
What happens when we're asking about present or past facts/events? For present facts, both do you know and did you know work, as stated above. For example, we could ask, Do you know that water boils at 100 degrees Celsius? or Did you know that water boils at 100 degrees Celsius? Again, did you know sounds slightly better to my ears.
What about the tense of the second verb when using did you know with true facts? Should we say Did you know that the earth is round? or Did you know that the earth was round? This is the same issue that often causes confusion for students who are learning reported speech (e.g., She asked if I like him vs. She asked if I liked him). The answer is that both are correct and acceptable. To me, using two past verbs in the same sentence sounds a little more natural and is what I'd be more likely to say.
For past facts/events, I would say that did you know is the better choice. For example, Did you know that the Titanic sank on April 15, 1912? makes sense, but Do you know that the Titanic sank on April 15, 1912? doesn't sound as good, at least to my ears. Note that both are technically correct, though. (Did you already learn this fact about the past? Do you know, at this present moment, about this past fact?)
We're sticking with Did you know as our heading of choice. For full sentences, though, we'll use Do you know (these words)? Do you agree with our decision? Let us know in the comments section below!