Among Vs Between
Among is used when more than two items are in question, between is used when there are only two items:
e.g. I have to choose between studying & watching a movie.
I have been admitted to several schools. I now need to choose the best among them.
Both Vs Each
Use "both" to point similarities, "each" to point differences. "Each" is always singular
Twice Vs Double
Use "twice, thrice" for comparison; "double, triple" is used as a verb only.
Each Other Vs One another
"each other" is for two things; "one another" for more than two.
If v/s Whether
If there is a choice between the two, use "whether". On GMAT, "if" is primarily for if/then type sentences.
like v/s such as
Use "such as" when possible. On GMAT. "like" is used when implying "similar to"