WHETHER AND IF
Many writers seem to assume that 'whether' is interchangable with 'if.' It isn't. 'Whether' expresses a condition where there are two or more alternatives. 'If' expresses a condition where there are no alternatives. E.g., I don't know whether I'll get drunk tonight. E.g. I can get drunk tonight if I have money for booze.
FEWER AND LESS
'Less' is reserved for hypothetical quantities. 'Few' and 'fewer' are for things you can quantify. E.g., the film has fewer than ten employes. E.g., the film is less successful now that we have only ten employes.
FARTHER AND FURTHER
The word 'farther' implies a measurable distance. 'Further' should be reserved for abstract lengths you can't always measure. E.g., i trew the ball ten feet farther then Bill. E.g., the financial crisis caused further implication.
SINCE AND BECAUSE
'Since' refers the time. 'Because' refers to causation. E.g., Since I quit drinking I've married and had two children. E.g., Because I quit drinking I no longer wake up in my own vomit.
DISINTERESTED AND UNINTERESTED
Contrary to popular usage, these words aren't synonymous. A 'disinterested' person is someone who's impartial. For example, a hedge fund manager might take interest in a headline regarding the performance of a popular stock, even if he's never invested in it. He's 'disintersted'. I.g., he doesn't seek to gain financially from the transaction he's whitnessed. Judges and referees are supposed to be 'disinterested'. If the sentence you are using implies someone who couldn't care less, chances are you'll want to use 'uninterested'.
DIFFERENT THAN AND DIFFERENT FROM
This is a tough one. Words like 'rather' and 'faster' are comparative adjectives, and used to show comparison with the preposition 'than', (e.g., greater than, less than, faster than, rather than). The adjective 'different' is used to draw distinction. So., when 'different' is followed by a preposition, it should be 'from', similar to 'separate from,', 'distinct from', or 'away from' e.g., My living situation in New York was different from home. There are cases where 'different than' is appropriate. If 'than' operates as a conjunction. E.g., Development is different in New York than Los Angeles. When in doubt, use 'different from'.
AFFECT AND EFFECT
Here's a trick to help you remember. 'Affect' is almost always a verb (e.g., Facebook affects people's attention spans) and 'effect' is almost always a noun (e.g., Facebook effects can also be positive). "Affect' means to influence or produce an impression - to cause hence, an 'effect'. 'Effect' may be used as a transitive verb, which means to bring about or make happen. E.g., My new computer effected a much-needed transition from magazines to Web porn. There are similarly rare examples where 'affect' can be a noun. E.g., His lack of affect made him seem like a shallow person.
IRONY AND COINCIDENCE
Too many people claim something is the former when they actually means the latter. For example, it is not 'ironic' that 'Barbara moved from California to New York, where she ended up meeting and falling in love with a fellow Californian.' The fact that they're both from California is a 'coincidence'. 'Irony' is the incongruity in a series of events between the expected results and actual results. 'Coincidence' is a series of events that appear planned when they're actually accidental. So, it would be 'ironic' if 'Barbara moved from California to New York to escape California men, but the first man she ended up meeting and falling in love with was a fellow Californian.'