OPTATIVE

Fundamental value: the user expresses the wish that the subject perform (etc.) the action.

a) Ἀπολυθεῖεν. May they be freed (or, more up to date: I wish they would be freed.) Πάντες εὐτυχοῖτε πάντα. May you all be happy in everything.

b) The addition of ἂν makes the wish even more modest and turns it into the expression of a possibility.

Ἀπολυθεῖεν ἄν. Perhaps they will be freed.

Ἀπίοιεν ἄν. Maybe they will go away. (the name for this is potentialis).

Often this type of main clause is accompanied by a condition: "Maybe they would leave, if we were to stop offering them drinks"; the verb in the secondary clause is also in the optative:

Εἰ φιλοφρόνως αἰτοίης, ποιήσαιμι ἂν τοῦτο. If you were to ask civilly, I might do this.

In Epic and other dialects one may encounter κέν ( or κά) instead of ἂν.

The above is sufficient for optative in main clauses, but

In secondary clauses  one encounters optatives in which little or nothing of the original meaning is traceable: in certain, but not all kinds of secondary clause the verb may be put in the optative when the main clause is in the past tense; one names these optatives finalis, iterativus or obliquus according to the type of secondary clause in which they appear:

Οἱ Λακεδαίμονες τοὺς παῖδας ἐπαίδευον μόνον ὅπως εὖ μαχέσοιντο. The Spartans raised their sons solely that they would fight well.

 Ἐκωλύομεν τοὺς πολεμίους μὴ διαβαῖεν. We tried to prevent the enemies from crossing.

Τὴν γεφύραν ἀνεῖλον (ἵνα) μὴ διαβαῖεν οἱ πολέμιοι. They demolished the bridge to prevent the enemies from crossing.

Ἐφοβούμεθα μὴ οἱ πολέμιοι διαβαῖεν. We were afraid that the enemies might cross.

 Ἀεὶ ἐλαμβάνετε ὅσα ἐπιθυμοῖτε. You always took as much as you desired.

 Ἔγνωσαν ὅτι οὔποτε πείσοιεν αὐτούς. They realized that they would never persuade them.

N.B. There is no rule that all secondary clauses have verbs in the optative. Certain kinds never take an optative. In  the first and last examples you have encountered a fut opt; there is no other use for fut optatives.

The difference between durative and aorist almost disappears in the optative!

The above treatment of the optative is not exhaustive; it was designed to cover a good majority of cases, and I trust will prove sufficient for understanding almost all one will encounter.