为什么 and 怎么: why, how come
代词 / pronouns (疑问)
Tone colorsā tone 1á tone 2ǎ tone 3à tone 4a neutral
Subject + 为什么 / 怎么 + (不/没) + Verb？
为什么 = "why": a neutral question about the reason. Answer with 因为.
怎么 can also mean "how come": it shows surprise or mild reproach: 你怎么没来？(How come you didn't come?).
怎么了？ = "what's wrong? / what's up?". Reminder: 怎么 + verb (no surprise) = "how to": 怎么走？(Which way do I go?)
Examples
你为什么不去参加聚会？
Nǐ wèi shénme bú qù cānjiā jùhuì?Why aren't you going to the party?
你怎么还没到？大家都在等你。
Nǐ zěnme hái méi dào? Dàjiā dōu zài děng nǐ.How come you're not here yet? Everyone's waiting.
你怎么了？不舒服吗？
Nǐ zěnme le? Bù shūfu ma?What's wrong? Are you unwell?
A：你为什么这么高兴？B：因为明天放假！
A: Nǐ wèi shénme zhème gāoxìng? B: Yīnwèi míngtiān fàngjià!A: Why are you so happy? B: Because tomorrow's a day off!
Common mistakes
为什么你不来？ is possible, but ✓ 你为什么不来？ (after the subject) is most natural.
✗ 你怎么了吗？ → ✓ 你怎么了？