Usage of Prepositions + Relative Pronouns

Question

I am going crazy because of prepositions + relative pronouns.

Here are some examples and please read and tell me if in the way that I have understood is right or not.

• (1) Do you know the date when we have to hand in the essay?

My grammar book says (1) can be changed more formally like (2):

• (2) Do you know the date on/by which we have to hand in the essay?

But I do not know if (3) has the same meaning as (1) and (2):

• (3) Do you know the date which we have to hand in in the essay on/by?

I don't know why I feel like (3) is weird. Maybe, because it is wrong?

It is really confusing because I have known that I can make Prepositions + which sentences when prepositions are used with certain verbs like this:

• Playing games in which I am interested are good for health.

• Playing games which I am interested in are good for health.

                **be interested in**
  

Answer 1

The process that puts prepositions (as well as other things, occasionally) in front of relative
pronouns is called Pied-Piping (honest), and it's very complex syntax, as you can see from the link.

In the case of these sentences, there are actually two different questions, depending on whether the essay can be handed in early or not. That's the difference between on, which refers to a single day, and by, which refers to an end date.

If the essay must be handed in on a particular date, then

• (1) is fine
• (2) is OK with on but not with by, which implies an end date for the hand-in period

• (3) is grammatical but hopelessly stilted; no native speaker would say this --
        we'd leave out the parts that aren't necessary:

• (4) Do you know the date we have to hand in the essay? OR

• (4') Do you know the date we have to hand in the essay by? (if there is a period with an end date)

or, more likely, we'd just use when

• (5) Do you know when we have to hand in the essay (by)? (by for an end date again)

Answer 2

(1) is incorrect. (2) and (3) do have the same meaning; the one you use is entirely up to your personal preference; you might feel that (3) is awkward because it ends with a preposition, and this is seen by some to be poor writing. Thus, (2) is the best option, though (3) is technically valid.

Your examples about "to be interested in" are both just as correct as the other.

Answer 3

Example (1) is awkward because “Do you know the date?” and “when” are basically the same question, so “When do we have to hand in the essay?” would be more natural.

Example (2) is technically fine, but extremely formal! I find it hard to imagine circumstances when I would use that level of formality while asking about a due date for class work.

Example (3) is okay but a bit clumsy. There is no problem with the preposition being at the end of the sentence, but the relative pronoun is an object and can be left out (“Do you know the date we have to...”) and it has too many words - “when do we have to...” is simpler.

Your first ‘playing games’ example is formal in register, and the second can lose the relative pronoun, but the main problem is that the subject is “playing games” so the verb should be “is”, not “are”.