When do you use a comma with as?

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The use of comma before the conjunction “as” is quite confusing. Are there any rules to use the comma correctly? Thank you.

Conjunction or Adverb?

Do not use a comma before “as” when using it in a comparison. In this usage, it’s an adverb. I’m as happy as a clam. It’s an adverb because it relates to the state-of-being verb. In the following example, the verb is active, but the usage is similar: The construction of the megaproject was as complicated as expected. (Notice here that “as” follows a verb; only use a comma after a verb for an appositive). Here’s an appositive form: The construction of the megaproject was complicated, as expected. Notice here that there is no longer a comparison, the whole phrase is adverbial, applied to complicated. In this case the usage is as a conjunction.

Use a comma with “As” as a conjunction

As a conjunction, “as” is equivalent to “because”. Do use a comma when “as” can be substituted with “because”. The megaproject went billions of dollars over budget, as the cost of cement was unexpectedly increased.

However, we do use a comma for the above when it’s written as an appositive.

Simultaneous time: The expense of the megaproject, as the government cut taxes, put a strain on the country’s finances.

Do not use a comma with “As” as an adverb

Do not use a comma when “as” is used for simultaneous time. This is an adverbial use. The megaproject went billions of dollars over budget as the government continued to claim it was successful.