Chapters 3 to 16 worked through the indicative, the mood that asserts. This chapter introduces one that does not.
The subjunctive presents an action as contingent — wanted, intended, possible, feared — rather than as fact. English has almost lost it; we keep a trace in "if I were you" and "God bless you". Greek uses it 1,856 times, so there is no reading around it.
The forms are the easy part
Genuinely easy, and there are only two tenses to learn.
1. The stem is the same as the indicative's.
2. There is never an augment, even in the aorist — because the subjunctive does not mark time.
3. The connecting vowel lengthens: ο becomes ω, ε becomes η.
4. The endings are the primary ones, in the aorist as well as the present.
So λύω, λύῃς, λύῃ, λύωμεν, λύητε, λύωσι(ν) in the present, and λύσω, λύσῃς, λύσῃ… in the aorist, where the σα has become σ before the long vowel.
The practical signal is a long vowel where you expected a short one, and no augment. That is most of recognising a subjunctive, and not all of it — as the next section says, λύσω is spelled exactly like the future indicative, and fifteen real verbs in the New Testament have the two forms identical. Where the spelling will not decide, the construction does.
The aorist is the default
Because the subjunctive marks no time, its tenses give aspect and nothing else — exactly as chapter 16 said. And the split is lopsided: of the 1,856 subjunctives in the New Testament, 1,387 are aorist and 459 present, three to one.
That matters for how you read them. The aorist is the unmarked choice, the one a writer reaches for by default, so it carries no particular weight. A present subjunctive is the marked one, and is where a writer has chosen to present the action as going on.
Ten perfect subjunctives exist in the whole New Testament. Ignore them until one stops you.
ἵνα, and purpose
The commonest subjunctive by far is the one after ἵνα, "in order that" — which occurs 662 times and is followed by a subjunctive almost every time.
οὗτος ἦλθεν εἰς μαρτυρίαν, ἵνα μαρτυρήσῃ περὶ τοῦ φωτός
"He came for a witness, in order that he might testify about the light." ὅπως does the same work 53 times, and ἵνα μή gives "lest".
A caution worth having early: ἵνα in Koine had already begun to spread beyond purpose into result and simple content — "that", introducing what somebody said or wanted. So ἵνα is not always purpose, and reading purpose into every one of the 662 will produce sermons the text does not support.
Let us — the hortatory subjunctive
In the first person plural, the subjunctive urges the speaker's own group to act. English needs "let us".
Ἀγαπητοί, ἀγαπῶμεν ἀλλήλους
"Beloved, let us love one another." Present, so the loving is presented as going on rather than as a single act — which is what lets "keep on loving" be a fair translation here. As chapter 3 said of the same aspect: it allows that reading, it does not require it.
δι’ ὑπομονῆς τρέχωμεν τὸν προκείμενον ἡμῖν ἀγῶνα
"Let us run with endurance the race set before us." Present again, and it suits a race, which is a thing you do over time. Notice that the durability is in ὑπομονῆς, "endurance", as much as in the tense — do not make the ending carry an argument the sentence is already making.
This is the construction to know for preaching, because it is how the New Testament says "we", including the writer in the command he is giving.
Do not start — μή with the aorist subjunctive
Greek prohibits in two ways, and they are not interchangeable.
μή with the aorist subjunctive generally forbids an action being begun.
μὴ οὖν μεριμνήσητε εἰς τὴν αὔριον
"So do not worry about tomorrow." The other way — μή with the present imperative — generally means stop something already going on, and that is the next chapter.
"Generally" is the important word. The contrast is a tendency in the aspect, not a rule, and it is regularly overstated from pulpits. Where the aorist is doing something beyond the default it usually adds urgency rather than a fine distinction about beginnings.
οὐ μή — the strongest negative in Greek
Two negatives together, with the aorist subjunctive, and they do not cancel. They intensify.
οὐ μὴ εἰσέλθητε εἰς τὴν βασιλείαν τῶν οὐρανῶν
"You will certainly not enter the kingdom of heaven." Render it "never" or "certainly not"; a plain "not" loses what the construction is for.
It is worth spotting because it is emphatic by construction rather than by context — one of the few places in Greek where the grammar itself, and not the sentence around it, is doing the emphasising.
Conditions
The subjunctive is also how Greek frames a condition that is open rather than settled. The marker is ἐάν — which is simply εἰ + ἄν — and it occurs 331 times.
ἐὰν εἴπωμεν ὅτι ἁμαρτίαν οὐκ ἔχομεν, ἑαυτοὺς πλανῶμεν
"If we say we have no sin, we deceive ourselves."
Set that against a condition with a past indicative, where the speaker treats the premise as untrue:
Κύριε, εἰ ἦς ὧδε οὐκ ἂν ἀπέθανεν ὁ ἀδελφός μου
"Lord, if you had been here, my brother would not have died" — εἰ with a past indicative and ἄν in the main clause, which is the contrary-to-fact pattern. Martha means: you were not here, and he did die.
The full set of classes is in Tables, and it is worth reading once you have met a few in the wild.
What to watch for
A long vowel and no augment is the signal. λύσῃ is subjunctive (or, as chapter 13 showed, future middle); ἔλυσε is indicative.
Some forms are ambiguous. The present active subjunctive first singular λύω is spelled exactly like the indicative, and the aorist subjunctive λύσω exactly like the future indicative. Only the construction around them decides.
The negative is μή, not οὐ — as chapter 3 said, the negative follows the mood rather than the meaning.
And ἵνα is not always purpose. It is the single easiest place in this chapter to overclaim.