Extra information gets a pair of commas. Information the sentence needs to make sense gets none. Delete the middle. If the sentence still says who or what you mean, the commas belong. That is one of the four comma jobs in ACT comma rules.
The deletion test
My coach, who ran track in college, stayed late. Remove the middle and you still know which coach. The clause is nonessential, so the commas come as a pair. One comma and no closer is always wrong.
The coach who ran track in college stayed late. Drop the middle and “the coach” could be anyone. The clause tells you which coach, so it is essential. No commas.
Names and which versus that
A name you already know is usually nonessential: Our captain, Lena Ortiz, called the play. A renaming phrase works the same way, which is the appositive case. Dashes can set off the same extra information with more emphasis. Those are in dashes on ACT English. The rest of the punctuation menu is in ACT punctuation rules.
The bottom line
Commas around a clause mean you could lift it out. If the sentence needs that clause to stay specific, leave the commas off.
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