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.)

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.