文頭の分詞句の場合、カンマは不可欠ではないように感じる。しかし，この位置の分詞句の直後にはカンマが置かれるのが通例である。表記習慣なのか、それとも「文法規則」なのか。

長い修飾句は副詞節の場合と同じように句読点を打たれるのが一般的である｡そうした修飾句が主節の前に位置する場合、その後にカンマが打たれる｡
To fully understand the impact of Einstein's ideas, one must be familiar with those of Newton, [infinitive phrase]
Leaning far out over the balcony, he stared at the waves below. [participle phrase]
In such a situation, speakers of the creole often deny their mother tongue. [prepositional phrase] --Peter Farb, Word Play
(『現代英語ハンドブック』11.2) （すでに第二章第５節で引用した箇所。）

Knowing that he wouldn't be able to buy food on his journey he took large supplies with him = As he knew etc.
Fearing that the police would recognize him he never went out in daylight = As he feared etc.
Being a student he was naturally interested in museums = Because/As he was a student etc.
(PEG, 277)（下線は引用者）

In experiments at Stanford University( , ) some scientists have succeeded in cloning human cells. [3a]
If you try hard( , ) you’ll succeed. [4a]
　……　文例[3a]と[4a]が示すように、カンマは、文頭に位置する多くの副詞要素の後では任意的であることが多い。しかし、カンマは、分詞節と無動詞節の場合には義務的となり、不定詞節[infinitive clauses]の場合はあるのが普通である。
Knowing my views, they refrained from discussing the subject.
Out of breath, he slumped down in the chair.
To keep the star moving so quickly and in such a small orbit, its invisible traveling companion would have to be at least nine times more massive than the sun.
(CGEL, Appendix�V.17) （下線は引用者）