Deleting Punctuation Around Essential Elements

Commas around a clause announce that the clause is extra: the sentence would mean the same thing without it. That works fine for a noun that is already fully identified, like one person's name. But when a noun is a whole group or a vague category, like 'students' or 'the ones,' the sentence needs some clause to narrow it down to the specific ones meant. Put commas around that narrowing clause and you accidentally say the opposite: that the fact applies to the entire group, with the clause tossed in as a side note.

That is exactly the trap in 'Students, who cheat, will be expelled.' Read it without the clause: 'Students will be expelled.' That now claims every student gets expelled, which is not what the sentence wants. Strip the commas instead, 'Students who cheat will be expelled,' and the clause clamps onto 'students' and narrows it to just the cheaters. That strip-it-and-check move, the deletion test, is the reliable way to decide: if removing the clause changes who or what is being talked about, no commas; if removing it only loses detail, keep them.

The word 'who' never decides this by itself. A 'who' clause can go either way depending on the noun in front of it. A vague plural noun like 'students' or 'the ones' needs the clause to pin down a meaning, so no commas. A noun that already names one unique thing, like the Eiffel Tower, needs no narrowing, so any clause about it stays extra and keeps its commas. This same logic links to the companion skill of adding commas around true interrupters: both rest on one question, does the sentence still identify the same thing without the clause.