as
Introducing a subordinate clause with as may cause ambiguity. Instead, use because to express a causal relationship and while or when to express a time relationship.
change As we were having lunch together, he decided to reveal his plans. [Ambiguous: Does the writer mean because we were having lunch together or merely while we were having lunch together?]
to Because we were having lunch together, he decided to reveal his plans.
or While we were having lunch together, he decided to reveal his plans.
as / like
Although in speech like is often used as a conjunction instead of as ("He thinks just like I do"), this usage should be avoided in all but the most informal writing. To introduce a dependent clause (remember that a clause includes both a subject and verb), use the subordinating conjunction as (or as if or as though) instead of the preposition like. (For more about introducing clauses with as, see as.)
He spoke exactly as I did. [The dependent clause I did is introduced by the subordinating conjunction as.]
but His accent was just like mine. [The pronoun mine is the object of the preposition like. ]
Like may be used in elliptical constructions that omit the verb from a clause.
She took to architecture like a bird to nest building.
If the omitted portions of the elliptical construction are restored, however, as should be used.
She took to architecture as a bird takes to nest building.
As and like can both function as prepositions; however, as expresses equivalency, and like expresses similarity.
He plays chess as a professional. [The sentence means that he is a professional chess player.]
He plays chess like a professional. [This sentence means that he plays as well as a professional but he is not one.]
