by jwinawer Sat Oct 04, 2008 6:34 pm
It is a good question, though to be honest my guess is that it goes beyond the scope of what would reasonably be tested on the GMAT. But I'll give it a shot:
Generally the difference between an adjective and an adverb is that an adj answers the question "what?" and an adverb answers the question "how?" (or when, where, etc.) "Engineers act childish" is probably acceptable. In this case, you are depicting WHAT engineers act AS (They act as children, or they act childish). Alternatively, if you wanted to depict the manner in which they act, you would say "Engineers act childishly". That is HOW they act. Either is OK. The same cannot be said for the man acting quickly. This can only mean that the manner in which he acts is quick (so, use the adverb, quickly). It would not make sense to say that "quick" is a description of WHAT he acts as (a quick man? but that seems weird).
This is my best answer without further research. But again, I think it is further than you need to go for the GMAT.