Most bad quiz questions are not too hard. They are ambiguous, or they have two defensible answers, or they turn on a word the writer did not realise was doing so much work.
One question, one answer
‘What is the longest river in the world?’ has two defensible answers depending on measurement. ‘Which river flows through Cairo?’ has one. Prefer the second, or specify: ‘by most measurements’. If you cannot state the answer in a few words without a caveat, the question needs rewriting rather than a footnote.
Difficulty should come from knowledge, not wording
A hard question asks something few people know. A bad question asks something many people know, phrased so they answer a different question by accident. Read your question aloud to someone who does not know the answer and ask them what is being asked. If they restate it differently from what you meant, rewrite it.
Check the answer, then check it again
Facts drift. Populations change, records get broken, countries rename their capitals. Anything numeric or superlative needs a current source, not a memory. Date the question in your own notes. A quiz reused a year later will contain at least one answer that has quietly stopped being true.
The final test is whether a losing team would accept the answer without argument. If you can imagine the objection, write the question differently.