男

男 works like a sticker you slap onto a person, never a thing on its own. English lets 'male' be a noun, so learners copy that shape straight into Chinese. The fix is one tiny character: add 的, and 男 becomes 'the male one'. 他是男的 is what real people say when asked about someone's sex. Want a full word instead? 男人 means a man, plain and simple. So never leave 男 naked at the end of a sentence.

Both words point at male students, but the register gap between them is huge. 男生 is the everyday word — it covers classmates, guy friends, even grown men in casual talk. 男学生 is precise, slightly stiff, and shows up mostly in written narration or official forms. Texting a classmate about the guys in your group? 男生 wins every single time. Say 男学生 out loud over lunch and you sound like someone reading a textbook aloud. “六七个男学生， 七八个女同学， 八九个好朋友。” (Six or seven male students, seven or eight female classmates, eight or nine good friends.) ── [《一两件新衣服》](/read/new-clothes-and-good-friends#s3)

男朋友 is glued together as one block, and that block means boyfriend. Drop a 的 in the middle and you slice the block open into a plain description. Now it just means a friend who happens to be male — zero romance attached. Natives hear the difference instantly, and the wrong one creates a genuinely awkward pause. The same habit keeps 男医生 and 男学生 glued together, no 的 in between.