Thanks to the inadequacy of schoolteachers across the English-speaking lands, most people today believe the words like and as to be synonymous. And to pick up where these part-time restaurant staff left off, your contemporary English-speaker further honors this simple way of life by heaving as off the boat altogether, preferring like for essentially all applications.
Contrary to popular scripture, however, like and as have discrete usages. To know the difference between these comparative words and employ these grammatical distinctions accordingly is to serve three main functions: to sharpen your logic, variegate your prose, and alienate yourself from friends.
The key distinction when comparing like and as is that like, in this case, is a preposition—an element of speech that defines the relationship between words—and as is a conjunction—an element of speech that connects words to other ones.
Therefore, like may be used only in direct comparison to a noun (phrase).
Noun to noun:
Angry, deformed, and horseless, he looked like King Richard III.
Notice that there is no verb after like. For direct noun comparisons using like, there is never a verb, not even implied, omitted verbs.
Noun-phrase to noun-phrase:
His angry gait, deformed spirits, and horseless demeanor made him look like Richard III pottering about after a rough day out at Bosworth.
In this noun-phrase to noun-phrase example, the verb “pottering” exists after Richard III, yet the comparison is still directly noun to noun: gait, spirts, demeanor to Richard III.
As, on the other hand, also connects and compares verbs.
Angry, deformed, and horseless, he paced as King Richard III did at the Battle of Bosworth Field.
Here, we’re still talking about our angry, deformed, horseless man, but we’re including, or creating a conjunction for, these two verbs, “paced” and “did.”
The same rules apply for similes, which typically use like, as, as if, or as though.
For the kingliest examples of similes in their proper forms, consider none further than the similes of Sir P.G. Wodehouse.
“There was a sound in the background like a distant sheep coughing gently on a mountainside”
Here, the like form is correctly comparing the two nouns “sound” and “sheep,” or the noun phrases “Sound in the background” and “a distant sheep coughing gently on a mountainside.”
“She looked as if she had been poured into her clothes and had forgotten to say ‘when.’”
Used just like as, as if creates a comparison that includes verbs, which, in this case, suggests the image of an open-minded woman who hadn’t been keeping her eye on the gravy boat.