1. What was your parents' dream?
Mother wanted to be a nurse, and Father wanted to be an architect.
2. What were our parents' dreams?
Mother wanted to be a nurse, and Father wanted to be an architect.
(Which question is corect? Do we have to use the singular form or the plural form?)
Number 2 is correct.
For number 1, you'd expect your parents' dream to be one they both had together. Your parents' dream may have been to live in a big house or to raise successful and happy children.
Both questions are grammatically correct, but they have slightly different meanings.
The first question, "What was your parents' dream?" implies that your parents had a shared dream or a common goal together. It suggests that they had a collective aspiration.
The second question, "What were our parents' dreams?" implies that each parent had their own individual dreams or goals, which might be different from each other. It suggests that there were multiple dreams among your parents.
So, depending on what you want to convey, you can use either the singular form ("What was your parents' dream?") or the plural form ("What were our parents' dreams?") to describe your parents' aspirations.