Using 怎么 to Ask "Why"
Up to this point, we have formally learned 怎么zěnme to express "how", as in "how do you get to Starbucks?" and 怎么样zěnmeyàng to ask about people's opinion, as in "What do you think about (a suggestion)?" We are going to explore here, some more common colloquial usages that vary from the meaning we know.
怎么zěnme+ （不bù/没有méiyǒu）+ verb phrase /adjective ？
When we use the grammar structure “怎么zěnme+verb phrase/adj. ” express an interrogative sentence， it has the same meaning with 为什么. Mostly it's used to ask why you didn't do something. There may be an accompanying feeling of surprise or strange.
| Nǐ shàng gè xīngqí zěnme qù zhōngguóle? ——> 你上个星期怎么去中国了？ Why did you go to China last week? |
| Nǐ jīntiān zěnme zhème shēngqì? ——> 你今天怎么这么生气？ Why are you so angry today? |
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Nǐ zuótiān zěnme méiyǒu lái xuéxiào? ——> 你昨天怎么没有来学校？ |