First and foremost, the comma before the word "as" if this Union is included in the phrase role in the sentence which is close to the Union said. These include: "purposely", "as a result" "as an exception", "usually", "as always", "I", "how now", "like" and others. For example: in the Evening, as luck would have it, the sun was shining. Another example: And now he, as a consequence, is in jail.
If the Union "as" is a connecting between the two parts of the complex sentence, the comma is placed. For example: I've been thinking how it happened that we got lost in the woods, knowing the way. Another example: He stared as the fire burns down and his eyes languished some sadness.
If there is a circumstance which is expressed the comparative turnover, starting with the Union "as", a comma must be put. Example: Her eyes were as blue as the clear sky. Another example: he was strong as a lion in mortal combat.
Pay attention that if after turnover with the Union "as" offer ends at the end of the turnover you need to put another comma. For example: I distinctly heard a baby crying, but did not get up. Another example: she was laughing like the happiest man and ran towards me.
If the turnover that contains the Union "as" acts in the role of circumstances of manner, to put a comma is not necessary. For example: he spake like an Englishman. In these cases, turnover can easily replace the adverb (in this case "English"). In some cases, the turnover is replaced by a noun in instrumental case. Unfortunately, it is difficult to distinguish the circumstances of manner to the circumstances of the comparison.
Also the comma should not be put if the Union "as" part of the idiom. However, there should be sure that this phrase is sustainable. Example: he sat in the chair in front of me on pins and needles.
If the offer without the Union "as" has a complete meaning and is part of the predicate, the comma also do not need to put. For example: he held as a wolf. Also, no comma should be placed, if the word "how" is between predicate and subject. If it wasn't for this Union, it would be necessary to put the dash. For example: water as a mirror (water mirror).
If before the comparative turnover is the negation of "not" or the particle "just", "exactly", "exactly-in-exactly", "like", "quite", "nearly", "quite", to put a comma is not necessary. For example, the eyes of Harry was just like Lily.