Can we use "who" by itself as a subject in spoken language?
For example, I want to say " Students who didn't come here yesterday are lazy." But I say " Who didn't come here yesterday are lazy" instead because I talk to students and I don't want to say "students" again. Does it make sense?
This is called a "headless relative clause" or "fused relative clause". In English, the pronouns who and whom cannot normally be used as fused relative pronouns:
The categories 'person' or 'people' are [...] so general in meaning that they normally take a further qualification, as in The people who came were all enthusiastic. [...] the head noun people is almost redundant; still, it cannot be left out in English, i.e. we cannot speak of *Who came were all enthusiastic. Obviously humans are too important to be left unmentioned as a head. The grammatical solution English has found is using relative clauses headed by the demonstrative pronoun those for plural humans (25a) and the quantifier anyone (25b) or the personal pronoun he (25c) as heads for singular humans:
- a. Those who say so are liars.
b. Anyone who says so is a liar.
c. He who says so is a liar.(Cognitive English Grammar, by Günter Radden, René Dirven, p. 162)
So it is not correct. You need a noun or pronoun before "who".
In this case, the pronouns that would work are "those" and "anyone".
"Those who didn't come here yesterday are lazy."
You can say this if you know some people didn't come."Anyone who didn't come here yesterday is lazy."
You should say this if you're not sure if there are some people who didn't come.
As MorganFR and 1006a said in comments,
- "Whoever didn't come here yesterday is lazy"
is also possible.