現場では、「比較級・最上級はmuch, by farで強める」といった安易な理解で終わっている参考書・問題集が多いように感じています。一例を挙げます。
「ずっと…」「はるかに…」のように比較級を強調する場合は、much, (by)far, still, even, a lotなどの副詞（句）を比較級の直前に置く。（『ヴィンテージ』）
最上級の前に (by)farやmuchを置いて、「ずばぬけて…」のように最上級を強調することができる。（『ヴィンテージ』）
しかし、ことはそれほど単純ではないのです。まずは最上級を修飾するmuchから見てみましょう。①～③をご覧ください。
①This is much the best of all. ②He is much the fastest runner. ③This book is much the most instructive.
このような用例を”old or stilted”と感じる英米人が多いのです。私たちの調査段階で、故ボリンジャー博士は次のようにコメントされました。
I believe your informant is right about much the best being somewhat old-fashioned, because while I have no difficulty with much the best itself, I find much the with other superlatives a bit odd, as in John is much the oldest, This string is much the longest, etc. By far presents no such problem. So your ② and ③ are odd for me.
今度は比較級を強調するby farです。引き続きボリンジャー博士のコメントをお読みください。④～⑥の何が問題点かがお判りでしょう。
④This book is by far better than that. ⑤Asia is by far larger than Australia. ⑥He drove his car by far more carefully than before.
By far with a directly following comparative degree I would not use at all, though I would accept it as correct. In the first place it is too strong, and in the second place if I wanted to be that strong I would use plain far instead (This book is far better than that). For me, by far is unnatural as a pre-adjunct. I want it to come later (This book is better than that by far).
