一昨日の記事「#5506. be to do 構文は古英語からあった」 ([2024-05-24-1]) に引き続き be to do 構文の歴史について．
この構文は古英語から観察されるが，伝統的な説として，ラテン語の対応構文のなぞりではないかという議論がある．その背景には，主にラテン語からの翻訳文献に現われること，そしてラテン語の対応構文と連動して受動態 (passive) の意味で用いられるのが普通であるという事実が背景にある．異なる説もあるので慎重に議論する必要があるが，ここではラテン語影響説を支持する Mustanoja (524--25) より，関連箇所を引用しよう．
PREDICATIVE INFINITIVE
INFINITIVE WITH 'To' AS A PREDICATE NOMINATIVE (THE TYPE 'TO BE TO'). --- The construction to be to, implying futurity and often obligation (the infinitive being originally that of purpose), is comparatively infrequent in OE and early ME, but becomes more common in later ME and early Mod. E. It probably arose under Latin influence, the infinitive with to being used to render the Latin future participle (-urus) or gerundive (-ndus), as in wæron fram him ece mede to onfonne ('aeterna ab illo praemia essent percepturi') and he is to gehyrenne ('audiendus est'). In OE this infinitive occurs mostly in a passive sense (he is to gehyrenne; --- hu fela psalma to singenne synt 'quanti psalmi dicendi sunt'). This usage continues in ME and even today. ME examples: --- þi luve . . . oþer heo is to ȝiven allunge oþer he is for to sullen, oþer heo is for to reaven and to nimen mid strengþe (Ancr. 181); --- that ston was gretly for to love (RRose 1091). The periphrastic passive appears in the 14th century: --- þey beþ to be blamed (RMannyng HS 1546).
In OE the predicative infinitive of this type is comparatively seldom found with an active meaning (wæron ece mede to onfonne 'aeterna praemia . . . essent percepturi'). This use continues to be rare in early ME, but becomes more frequent later in the period. In many instances the implication of futurity is faint or practically non-existent: --- þes fikelares mester is to wrien ant te helien þet gong þurl (Ancr. 36); --- al mi rorde is woning ant to ihire grislich þing (Owl&N 312); --- myn entencioun Nis nat to yow of reprehensioun To speke as now (Ch. TC i 685). In many instances, however, the implication of futurity is prominent: --- Crystes tresore, Þe which is mannes oule to save, as God seith in þe Gospel (PPl. B x 474); --- whan the sonne was to reste, so hadde I spoken with hem everichon (Ch. CT A Prol. 30). There are scholars who believe that this construction is used as a real future equivalent in late ME.
不定詞（あるいはより一般的に非定形動詞）が，形式上は能動態的であっても意味上は受動態的に用いられる件については，以下の記事を参照されたい．
・ 「#3604. なぜ The house is building. で「家は建築中である」という意味になるのか？」 ([2019-03-10-1])
・ 「#3605. This needs explaining. --- 「need ＋動名詞」の構文」 ([2019-03-11-1])
・ 「#3611. なぜ He is to blame. は He is to be blamed. とならないのか？」 ([2019-03-17-1])
・ 「#4104. なぜ He is to blame. は He is to be blamed. とならないのですか？ --- hellog ラジオ版」 ([2020-07-22-1])
引用にもある通り，この be to do 構文は後に能動態的にも用いられるように拡張し，現在に至っている．
・ Mustanoja, T. F. A Middle English Syntax. Helsinki: Société Néophilologique, 1960.
英語には be to do 構文というものがある．be 動詞の後に to 不定詞が続く構文で，予定，運命，義務・命令，可能，意志など様々な意味をもち，英語学習者泣かせの項目である．この文法事項については「#4104. なぜ He is to blame. は He is to be blamed. とならないのですか？ --- hellog ラジオ版」 ([2020-07-22-1]) で少し触れた．
では，be to do 構文の歴史はいかに？ 実は非常に古くからあり，古英語でもすでに少なくとも「構文の種」として確認される．古英語では高頻度に生起するわけではないが，中英語以降では頻度も増し，現代的な「構文」らしいものへと成長していく．近代英語にかけては，他の構文とタグも組めるほどに安定性を示すようになった．長期にわたる文法化 (grammaticalisation) の1例といってよい．
先行研究は少なくないが，ここでは Denison (317--18) の解説を示そう．長期にわたる発達が手短かにまとめられている．
11.3.9.3 BE of necessity, obligation, or future
This verb too is a marginal modal, patterning with a to-infinitive to express meanings otherwise often expressed by modals. For Old English see Mitchell (1985: §§934--49), and more generally Visser (1963--73: §§1372--83). One complication is that the syntagm BEON + to-infinitive is used both personally [158] and impersonally [159]:
[158] Mt(WSCp) 11.3
. . . & cwað eart þu þe to cumenne eart . . .
. . . and said are(2 SG) you(2 SG) that (REL) to come are(2 SG)
Lat. . . . ait illi tu es qui uenturus es
'. . . and said: "Are you he that is to come?"'
[159] ÆLS I 10.133
us nys to cweðenne þæt ge unclæne syndon
us(DAT) not-is to say that you unclean are
'It is not for us to say that you are unclean.'
In Middle and Modern English this BE had a full paradigm:
[160] 1660 Pepys, Diary I 193.7 (5 Jul)
. . . the King and Parliament being to be intertained by the City today with great pomp.
[161] 1667 Pepys, Diary VIII 452.6 (27 Sep)
Nay, several grandees, having been to marry a daughter, . . . have wrote letters . . .
[162] 1816 Austen, Mansfield Part I.xiv.135.30
You will be to visit me in prison with a basket of provisions;
Visser states that syntagms with infinitive be are 'still current' (1963--73: §2135), but although there are a few relevant examples later than Jane Austen (1963--73: §§1378, 2135, 2142), most are of the fixed idiom BE to come, as in:
[163] But they may yet be to come.
まとめると，be to do 構文は古英語からその萌芽がみられ，中英語から近代英語にかけておおいに発達した．一時は他の構文とも組める統語的柔軟性を獲得したが，現代にかけてはむしろ柔軟性を失い，統語的には単体で用いられるようになり，現代に至る．
・ Denison, David. English Historical Syntax. Longman: Harlow, 1993.
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