怎么

怎 zěn is a relatively late character, built as 乍 zhà (suddenly, at first) over 心 xīn (heart). The heart radical marks it as a word of mind and feeling, and it has meant "how, in what way" in vernacular Chinese for many centuries, the puzzled turn of a thinking heart. 么 me (also written 麼) is a light suffix that attaches to question words, the same 么 in 什么 shénme (what) and 怎么 itself.

So 怎么 reads, morpheme by morpheme, as roughly "how-what", a doubled question particle that became the ordinary spoken word for "how". It belongs to a small family of vernacular interrogatives, 怎样 zěnyàng, 怎么样 zěnmeyàng, 怎么着 zěnmezhe, all spun from the same 怎. In more literary or northern speech you may also hear 咋 zǎ, a contraction of the very same idea.

The crowded corner here is "how" versus "why". 怎么 zěnme is first of all "how, in what way", a question about method that 为什么 cannot ask: 怎么做 ("how to do it"), never 为什么做 for that sense. When 怎么 does shade into "why", it is the surprised "how come": 你怎么不吃？("how come you're not eating?", noticing with some concern). 为什么 wèishénme is the neutral "why", a plain request for the reason: 你为什么不吃？("why aren't you eating?"). Choose 为什么 to genuinely ask a cause, 怎么 when you are also reacting to the unexpected.

The other pair is 怎么 versus 怎么样. 怎么 leans on a following verb and asks about manner (怎么去, "how to go"); 怎么样 stands more on its own and asks about a condition or an opinion (去得怎么样, "how did the trip go"; 这个怎么样, "how about this"). A quick test: if you could answer with a method ("by bus", "like this"), use 怎么; if you would answer with an evaluation ("pretty good", "not bad"), use 怎么样.