プログレッシブ英和中辞典
for〔前にコンマ・コロン・セミコロンを置いて〕((形式))というのも...，なぜかというと...だから；〔独立した文を導いて〕というのは［も］（◆補足的に理由を述べる語でbecauseより意味は弱い）
- It is morning, for the birds are singing.（朝だ、鳥が鳴いているもの。）
新英和中辞典
for 接続詞[通例コンマ，セミコロンを前に置いて，前文の付加的説明・理由として] という訳は...だから《★【用法】 文語的で会話中では用いない》.
- It will rain, for the barometer is falling.（雨が降るだろう、晴雨計が下がっているから。）
よく注意点として言われるのは、becauseと同じ調子で文頭に「For Ⓢ Ⓥ」を持ってきてはいけないということです：
- ✅It is morning, for the birds are singing.
- ✅It is morning, because the birds are singing.☞非制限用法。
- ❌For the birds are singing, it is morning.
- ✅Because the birds are singing, it is morning.
FANBOYS is a handy mnemonic device for remembering the coordinating conjunctions: For, And, Nor, But, Or, Yet, So. These words function as connectors. They can connect words, phrases, and clauses, like this:
- Words: I am almost dressed and ready.
- Phrases: My socks are in the living room or under my bed.
- Clauses: They smell really bad, so they will be easy to find.
andやbutと同じ等位接続詞であれば、because節と同じ調子で文頭に「For Ⓢ Ⓥ」を持ってくるのがおかしいのも理解できます。「前にカンマを入れる」というのも等位接続詞ならうなずけます（becauseなどの従属接続詞の前にもカンマを入れることはできますが、非制限用法になりカンマ無しの場合とは意味が変わります）。
ちなみに、文頭で等位接続詞を使用すること自体は誤りではありません：
[H]as it ever been wrong to begin a sentence with and or but? No, it has not. We have been breaking this rule all the way from the 9th century Old English Chronicle through the current day.... The 1959 edition of Strunk and White's The Elements of Style begins two sentences in a row with these prohibited words, and does so with nary a trace of self-consciousness.
- But since writing is communication, clarity can only be a virtue.
- And although there is no substitute for merit in writing, clarity comes closest to being one.
「For Ⓢ Ⓥ」も、単独で用いるのであれば（つまり「For Ⓢ Ⓥ, Ⓢ Ⓥ.」のような形でなければ）、普通に用例がたくさんあります：
- ✅For it is not my intention to confuse you.
- ❌Because it is not my intention to confuse you.☞従属節は単体では文を形成しない。
多くの辞書では、なぜか語の分類としては「接続詞」とあるだけで、等位なのか従属なのかまでは記載されていません。Collins Dictionaryではなんとforが従属接続詞として記載されています：
for 11. SUBORDINATING CONJUNCTION
You can use for to introduce a clause which gives the reason why you made the statement in the main clause. [literary]
- He had a great desire to have a home of his own for he had always lived with my grandmother.
ＣＧＥＬを調べたら、どうもforは従属接続詞か等位接続詞か微妙な、中間的（gradient、intermediate）なケースのようです。
A Comprehensive Grammar of the English Language
2.60 The simplest cases of gradience to identify are those relating to word classes. We shall see, for example, in 13.18f that there is a scale relating coordinating and subordinating conjunctions, such that and and if represent clear cases of each category, whereas for is in an intermediate position.... Nevertheless, for is closer to if in its syntactic behaviour than to and (cf 13.18f), and can reasonably be classed as a peripheral subordinator.
13.5 We regard these conjunctions as clearly coordinators: and, or, and but. And and or are central coordinators, but but differs from them in some respects. On the gradient between 'pure' coordinators and 'pure' subordinators are for and so that (meaning in this chapter, unless otherwise stated, 'with the result that')..... We class the conjunctions for and so that as subordinators in this book....
ＣＧＥＬも基本はforを従属接続詞（subordinator）として扱っていますが、より等位接続詞に近い性質として二点を指摘しています。一つは「置き換え（transposition）」ができない点です。等位接続詞はその前後の節の順番を逆にして入れ替えることが不可で、従属接続詞は入れ替えが可ですが、forは等位接続詞と同じく入れ替え不可です。
- ✅I am poor, but I am happy.
- ❌But I am happy, I am poor.
- ✅I am happy although I am poor.
- ✅Although I am poor, I am happy.
- ✅I am happy, for I am rich.
- ❌For I am rich, I am happy.
A Comprehensive Grammar of the English Language
13.8 Clauses beginning with and, or, and but are sequentially fixed in relation to the previous clause, and therefore cannot be transposed without producing unacceptable sentences, or at least changing the relationship between the clauses:
- ✅They are living in England, or they are spending a vacation there.
- ❌Or they are spending a vacation there, they are living in England.
- ❌Nevertheless John gave it away, Mary wanted it. [1a]
- ✅Although Mary wanted it, John gave it away. [1b]
- ❌For he was unhappy, he asked to be transferred.
- ✅Because he was unhappy, he asked to be transferred.
また、等位接続詞がその前に別の接続詞を置くことが不可で従属接続詞の場合は可なのに対し、forも等位接続詞と同じく前に別の接続詞を置くことはできません。
- ✅He is poor, but however, he is happy.（接続副詞）
- ❌He is poor, and but he is happy.（等位接続詞）
- ✅He was happy, because he was rich and because he had a lot of friends.（従属接続詞）
- ❌He was happy, for he was rich and for he had a lot of friends.
A Comprehensive Grammar of the English Language
13.10 And and or do not allow another conjunction to precede them. This is also true for but, for, and so that (of which the latter two will be treated as subordinators; cf 13.18f). On the other hand, subordinators as well as conjuncts can usually be preceded by conjunctions:
- He was unhappy about it, and yet he did as he was told.
- ✅He asked to be transferred, because he was unhappy and because he saw no prospect of promotion. [1]
- ✅She saved money so that she could buy a house, and so that her pension would be supplemented by a reasonable income after retirement. [2]
- ❌He was unhappy about it, and but he did what he was told.
- ❌He asked to be transferred, for he was unhappy and for he saw no possibility of promotion.
- ❌We paid her immediately, so (that) she left contented and so (that) everyone was satisfied.
