1. Explain it simply
为什么 means 'why'. Put it after the subject and before the verb: 你为什么…？
Answer with 因为 ('because') at the start of your reason.
A 为什么 question usually does not take 吗.
2. Show examples
你为什么喜欢北京？
Nǐ wèishénme xǐhuan Běijīng?
Why do you like Beijing?
为什么 before the verb
因为天气很好。
Yīnwèi tiānqì hěn hǎo.
Because the weather is nice.
因为 starts the reason
你为什么学汉语？
Nǐ wèishénme xué Hànyǔ?
Why do you study Chinese?
subject + 为什么 + verb
3. Share a tip
为什么 already makes it a question, so don't add 吗. The reason after 因为 comes as a full clause.