CONTINUOUSLY and CONTINUALLY: don't stop
Guideline. Continually is preferably used for conditions of time and continuously for conditions of space.
Common usage often employs continuously in both instances but keeping the distinction can help clarity and heighten impact.
PRESENTLY: is to come
Guideline. Presently is often used to mean "at present" or "currently" but its proper meaning is "in the near future" or "soon." In Great Britain presently retains its meaning of "in a short time." If presently is made a synonym of "currently" and "at present" then the flexibility and range of all these words are lost.
EACH OTHER and ONE ANOTHER: one or t'other
Guideline. Use each other to refer to two persons or things; one another for more than two.
IF & WHETHER: Chances are
Guideline. Use if when you're expressing a single condition. Use whether when the condition involves two possibilities. If you're using whether, you can usually omit the or not. Exception: when you want to give equal emphasis to both possibilities.
IN BEHALF OF & ON BEHALF OF: Interested parties
Guideline. Use in behalf of when your actions are designed to benefit somebody. Use on behalf of when you are acting as somebody's representative.
LIABLE & LIKELY: Looking ahead
Guideline. Use likely when you're dealing strictly with likelihood. Use liable when that likelihood involves a possibly unhappy outcome.
Source: Jim Engell, professor of English at Harvard