Hello , 问题 It could mean : - a question you ask - a problem - an issue or topic to discuss 难题 means a difficult problem . If you examine the word parts separately, it will be easy for you to detect the difference in meaning as follows: 题 could mean topic, problem of discussion ,a subject or an exam question 问 means to ask 难 means difficult or problem.

问题has several meanings: It could mean question/problem, for example: 提出问题（ask questions）/解决问题(solve the problem)。 If the question is difficult to answer,or the problem is difficult to solve, we call call it难题（难回答的问题/难解决的问题）,you can say提出难题/解决难题，For example： 昨天老师出的那条难题你有答案了么？Do you have the answer to the difficult question that teacher released yesterday? 他们面临资金短缺的难题（They are facing the tough problem of capital shortage) And 难题is always used after the verb出/提出，that is 出难题/提出难题 你总是给我出难题You alway give me big nuts to crack. And another meaning of 问题is trouble/wrong, it is usually used in the phrase出问题, 我的电脑出问题啦（There is something wrong with my computer） But we never say我的电脑出难题啦even if there are big bugs or damages with the computer. And 问题could also mean"the key point/the cause", for example 他总是输，问题就在于他没有信心（He always loses, the problem lies in that he doesn't have confidence）and难题can not be used here.

wen ti de yi si shi ni bu zhi dao huo shi bu li jie ti wen zhe suo xiang zhi dao de da an ! nan ti de yi si shi dui ni lai shuo ,ni yao hua hen da li qi qu jie jue ni yu dao de wen ti,huo zhe gen ben jie jue bu liao de wen ti! if you can read these Pinyin, it is not a problem, if you cant understand what do these Pinyin mean,ok, you have a very difficult problem!

Cheery is a Chinese expert! 厉害！

问题，it could be an easy quesion or diffcult one,but难题，it means hard to work it out,soo diffcult