"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 groß ist? (What is big?)
- 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."