"What something is?" vs "what is something?"

Question

I can write "what a cup is?" as question and I can write "what is a cup?". Are both forms grammatically correct?

The former one looking a bit off when used without context, but I'm planning to use it in a pair like: "What a cup is not?" and "What a cup is?". Stressing on IS rather than on a cup in this case, as I want to stress that definition is important and important when done in comparison to a not part.

Maybe it is sound even better with some adjective rather then noun, like: "What flexibility is not?" vs "What flexibility is?".

But I feel that this is somehow wrong still.

Answer 1

What is a cup? is a question form.

I can tell you what a cup is. is a statement form.

The question in writing always requires the inverted form.

Title: What a Cup Is and What It Is Not.

OR What Is or Is not a Cup

Answer 2

"OP asked if these four questions are grammatically correct in English:

1.What a cup is?
2.What is a cup?
3.What flexibility is not?
4.What flexibility is?

Among these, only question 2, "What is a cup?', is grammatically correct.

For questions 3 and 4, the correct form is :

• What is flexibility? (or Define flexibility)

In English, the Wh question follow this structure:
Wh-word + Verb + Subject/Object.

In contrast, German allows more flexibility in word order. Both constructions are grammatically correct:

• Was ist die Tasse? (What is the cup?)
• Was die Tasse ist?

Additionally in German.
'Was + Adjective + Verb' construction works well with adjectives, as seen in:

• Was schnell ist? (What is fast?)
• Was schön ist? (What is beautiful?)
• Was wichtig ist? (What is important?)

As I have learnt some German (through a correspondence course) I understand the context behind the question.
However, it's essential to recognize that German grammar rules cannot be directly applied to English."