Good question, Laura! I wondered whether anyone would point that out. There are several questions here.

1. Is 'For whom is the cake you made?' correct?

Hmm, just about. But it sounds very strange.

2. How about 'For whom did you make the cake?' Isn't that more correct than 'Who did you make the cake for?'.

Well, yes, it is technically correct, but..

2. Does anybody use this construction?

No. It's technically correct, but sounds awkward and unnatural. A native English speaker wouldn't ask a question in this way.

This isn't about the verb 'happen'. The verb 'happen' is an ordinary verb that works in exactly the same way as other verbs. It's all about subjects and objects.

When you ask most questions, the word order needs to be as follows - (Q) ASV. For example, with the question 'What are you doing?' : 1. Q -  Question word - What 2.  A - Auxiliary verb - are 3. S - Subject - you 4. V - Verb (main verb) - doing?

Simple tenses don't have auxiliary verbs, so we need to insert the auxiliary verb 'do' in the question form so that there is something to put before the subject. For example, not 'Where you live?' but 'Where DO you live? or 'What DID you have for breakfast?'.

This is why we say 'When did it happen?'

1. Q - When

2. A - did

3. S - it

4. V - happen?

In this question, the word 'it' is the subject. The word 'it' represents whatever it was that happened. For example 'When did the accident happen?'

However, when we say 'What happened?' the word 'What' represents 'the accident', making it also the SUBJECT of the sentence. 

1/3 Q/S - What

4 - V - happened?

In this case, there is no need to use an auxiliary verb. 'What happened?' 'An accident happened.'

It's the same as the difference between these two question forms

<em>'Who made the cake?'  'I made it'</em>

<em>'Who did you make the cake for? 'I made it for my grandma. It's her birthday.'</em>

In the first question 'Who' represents the subject (I), so there is no auxiliary verb. In the second, 'Who' represents an indirect object (the person the cake was for) , so you need an auxiliary verb.

So, to answer your original question, we say

<em>'When does it happen?'</em> and '<em>When did it happen?',</em>

but

<em>'What happens?</em>' and '<em>What happened?'</em>

This is because in the second set of questions, the word 'What' is the SUBJECT.

Thanks Su.Ki. again!

You are so kind and nice ...

And thanks Laura for your nice question :)

Thank you so much Su.Ki! I wondered if that was correct or not "for whom" as it is similar to the way we would say that sentence in Spanish :) I write down your answers and of course I thank you for your help and Meh for this entry :)

It's amazing the answer Su.Ki wrote! But from this another question came up to my mind, would be correct to say:

FOR WHOM IS THE CAKE YOU MADE? instead of WHO DID YOU MAKE THE CAKE FOR?