What's the difference between لِمَا and لِمَ?
What's the difference between لِمَا and لِمَ?
I know they both mean "why" but what would be the difference?
jazakum allahu kharan
جزاكم الله خيرا

Question

Answer

"لِمَ" أداة استفهام بمعنى لماذا (why)
مثال
- لِمَ تذهب الى النادي
* كي امارس الرياضة
"لِمَا" كلمة مكوَّنة من اللاّم وهي حرف جرّ للتعليل ، و ( ما ) اسم موصول بمعنى الذي
مثال
{ وَإِنَّهُ لَذُو عِلْمٍ لِمَا عَلَّمْنَاهُ }
"لما" لها معاني كثيرة تختلف حسب التشكيل والسياق 
useful link: http://www.almaany.com/ar/dict/ar-ar/%D9%84%D9%85%D8%A7/

Answer

لم  and   لماذا  =  Why?, what for?, for what reason?, for what purpose? 
Meaning would be given as per the context.

لما   alone will not be used for  question but,  here  "ما" will be used as relative pronoun.

Example :       لما في ذلك عيب  =  Because of defect in it.

Hope it will help you to understand the difference.

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