subject questions

[forum_mail]

Should I say :

1. What you liked about the course and what you didn't ?

or

2. What did you like about the course and what you didn't ?

I never know when I can omit the operator... like did and so on...

[Casiopea]

The following are grammatically correct:

[1] You liked what about the course?
<-d is a past tense marker>

[2] What did you like about the course?
<did carries the past tense marker>

The auxiliary "did" is needed. It carries the past tense marker.

[forum_mail]

yeah, but take a look at such (an ???) example :

Who made this cake ?

We don't say "Who did make this cake" do we ? Thus, I was thinking that mabye it works somehow the same with "What you liked..." but I guess that it doesn't ask about the subject.

[Tdol]

We can use 'who did make this cake?':
'Did you make this cake?'
'No.'
'Did James make this cake?'
'No.'
'Who did make this cake then?'

It's an emphatic form that would need a reason to be used.

[Casiopea]

In addition,

Replacement
Max made this cake.
Who made this cake.

"Who" replaces the subject "Max". There's no need for DO-insertion. "Who" hasn't moved anywhere. It remains in the subject position. The same holds true for objects. If the WH-word stays in the same position as the noun it replaces, then DO-insertion isn't necessary:

Replacement
Max made this cake.
Max made what?

Let's look at what happens when a WH-word is moved out of its position.

DO-insertion
Max made what? <replacement; grammatical>
What Max made? <WH-movement; ungrammatical>
What did Max make? <DO-insertion; grammatical>

Here's the rule: DO (i.e., do, does, did) is inserted iff the WH-word (e.g., what) is moved to the head of a clause that functions as a question. (See note, example 4. below)

Now let's look at the reason these are ungrammatical:

1. What you liked about the course and what you didn't?
2. What did you like about the course and what you didn't?

In 1., "What" represents the verb's object;i.e., You liked what about the course?, and that WH-word has been moved, which means that DO-insertion is needed. Note that, DO-insertion is needed directly after the WH-word, as in 3. below, not after the subject as in 1. and 2.:

3. What did you like ... and what didn't you like ... ?

1. What [did] you liked about the course and what *you didn't.
2. What did you like about the course and what *you didn't?

===
Note, the following structure doesn't require DO-insertion. It's perfectly grammatical without inserting "did":

4. Say what you liked about the course.

The reason being, 4. is not a question. It's a statement. The rule for DO-insertion holds that DO is inserted iff the WH-word is moved to the head of the main clause. In 4. "what" moves to the head of its own clause, not to the head of the main clause. It doesn't head the main clause. The imperative verb "Say" heads that clause.

As tdol mentions, you can insert DO in structures like 4. but its function or role is purely emphatic. In 5. below, "did" is not connected to the main verb phrase (i.e. "Say"). And the reason it follows the subject "you", and doesn't come directly after the WH-word "what".

Emphatic "did"
5. Say what you did like about the course? <main verb: imperative "Say">

DO-insertion
6. Say, what did you like about the course? <main verb: like>

Hope that helps.