The word optative - pronounced with the stress on either the first or the second syllable (IPA: /ˈɒp tə tɪv or ɒp ˈteɪ tɪv/) - comes, through the adjective optativus, from the Latin verb optare, ‘to wish for’, ‘to choose’. Optative is used both as an adjective and as a noun. As an adjective, it means ‘expressive of or related to choice, preference, or wish’, while as a noun it refers to the optative mood, a mood of the verb not found in English or any Germanic language but present in Ancient Greek and in Sanskrit.
In Ancient Greek the various forms of the optative mood of a verb have endings which distinguish them from the corresponding forms of the indicative and subjunctive moods. For example, while the first person singular of the present indicative and subjunctive of the verb λύειν (luein, ‘to loosen’) is λύω (luo), i.e., the ending is -ω (-o), the corresponding form of the optative is λύοιμι (luoimi), i.e., the ending is -οιμι (-oimi). The optative mood in Ancient Greek has distinctive uses. In a main clause it is used, as its name would imply, to express wishes – when it has this function it is sometimes preceded by εἴθε (eithe, ‘if only’) - but it is also used in certain types of subordinate clause if the verb in the main clause is in a past tense - e.g., in subordinate clauses after verbs expressive of purpose and verbs expressive of fear. (When the verb in the main clause is in the present or future tense, the verb in subordinate clauses of these types is in the subjunctive mood.)
The Greek phrase εἴθε γενοίμην (eithe genoimēn) in Rupert Brooke’s poem The Old Vicarage, Grantchester has its verb (γενοίμην, genoimēn) in the optative: γενοίμην, (genoimēn) is the first person singular of the present tense of the optative of γίγνεσθαι (gignesthai, ‘to be’, ‘to become’). The phrase means ‘Would I were’ [in Grantchester] - as Brooke immediately translates it.