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Only when your mental energy is needed elsewhere and you are caught off guard, however, do these emotions and feelings emerge.

Why should the "do" be placed ahead of the clause?

Could anyone please give me some hints?

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Explaining the use of "however" is fairly easy. It's there to emphasize that something (call it THING 1) is being contrasted with something else (THING 2). Here, THING 2 is the situation where, "your mental energy is needed elsewhere and you are caught off guard". We'd need more context to know exactly what is THING 1 -- which will have been mentioned or implied earlier in the text" -- but possibilities are situations where:

  1. Your mental energy is needed elsewhere but you are not caught off guard, OR
  2. Your mental energy is not needed elsewhere but you are caught off guard, OR
  3. Your mental energy is not needed elsewhere, nor are you caught off guard,

OK, now to explaining the significance of the word "do". For sure that word "do" is needed, but I'm not sure I can explain why. However, you also asked for hints, so in that light here is an example of the sorts of things I (a native, but non-linguistically-trained English speaker) do when trying to debug puzzles like this. It might be useful while you wait for one of our grammar ninjas to give a more precise answer.

So, first I'd notice that we can remove a chunk of words in your original phrase and still preserve the relevance of the question about the use of "do". So instead of:

Only when your mental energy is needed elsewhere and you are caught off guard, however, do these emotions and feelings emerge.

we could work on the much simpler:

Only when you are caught off guard do these emotions emerge.

The same question arises -- why do we need "do" -- but we're not distracted by the various other clauses (and we've already dealt with the "however"). And let's just check that:

Only when you are caught off guard do these emotions emerge.

Yes, that's fine -- same as your original. But, remove the "do" and:

Only when you are caught off guard these emotions emerge.

That is not fine -- again, just as with the original

So far so good; things are a bit simpler. Now let's run a couple of experiments. First, I'll see what happens if I remove that first word, "Only" and then mess with the "do":

When you are caught off guard these emotions do emerge.

Hmm, that's not actually wrong -- I could come up with some contexts where it would be OK -- but it's quite stilted. I'd probably avoid it. And then:

When you are caught off guard these emotions emerge.

Interesting; that one is actually fine. So with the initial "Only" removed, the "correctness" of the "do" word is reversed.

Second experiment. Put the "Only" back, and then add "It is" before it.

It is only when you are caught off guard do these emotions emerge.

Oh, no, no. That's just wrong. You'd have to change the "do" to "that" to get it to work (although you could then shove the "do" back in, just before "emerge", although that wouldn't be necessary). And then:

It is only when you are caught off guard these emotions emerge.

Ooh, that's a close one! It would be much better to put the "that" in before "these", but in this context I think you might just about get away with omitting it. So as before, this change (adding the "It is") makes the "do" wrong" and the non-"do" version OK (kinda).

So, to summarize:


      Only when ... <with the "do">    : OK 
      Only when ... <without the "do"> : NOT OK

           When ... <with the "do">    : NOT OK (ish) 
           When ... <without the "do"> : OK

It is only when ... <with the "do">    : NOT OK 
It is only when ... <without the "do"> : OK (ish)

And from that we conclude...I've no idea! I know that your original does need the "do", but I can't explain why. It's the classic "I just know in my knower!" problem of the untrained native speaker.

Well, you did ask for hints!

< cue the grammar ninjas >

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