Which of these sentence is correct? subjunctive or not?
“Unless he be mean, I will help him.”
“Unless he is mean, I will help him.”
Do we use the subjunctive mood? What are the verbs that require this mood? Is “unless” always followed by this mood?
(Not a native speaker for that matter***)
Solution 1:
In general we'd use the indicative ("is") in cases like this in modern English, after "unless" of "if". A modal "should" is also possible: "Unless he should be mean, I'll help him", or "if he should be mean, I won't help him".
Strictly, the present subjunctive ("be") is possible, but it sounds formal or archaic. I think it might be more common in American academic style than British English, where sounds archaic, affected or possibly legal. (See https://www.englishclub.com/grammar/subjunctive.htm - "be after if")
The background here is that the present subjective in contexts like these was more common in the past. Shakespeare gives us "If music be the food of love, play on", or this less famous example with unless:
Jack Cade: The proudest peer in the realm shall not wear a head on his shoulders, unless he pay me tribute;
(Henry VI, Part II: act IV, scene7, line 2735)
"Pay" is subjunctive here: the indicative, "pays", would be normal in contemporary English.
Google gave me a more recent example from a 1948 speech by General Douglas MacArthur:
“No man is entitled to the blessings of freedom unless he be vigilant in its preservation.”
As Jason Bassford mentions, the past subjunctive ("Unless he were mean") is also possible in this context in contemporary English, but with a hypothetical meaning: meanness is thought unlikely or impossible.
To my own (native British English) ear, something like "Unless he be willing to work for free, I won't help him" is just about possible, but it suggests formality and / or incredulity, whereas "unless he is willing to work for free, I won't help him" is matter of fact. "If need be" is still a widely used fixed expression: so "If need be, I'll help", and I suppose by extension "Unless need be, I won't help".
But in general, as a non-native, and unless you're sure, use the indicative.