Who is the subject, while whom is the direct object. What? It's possible to memorize a rule, but it's simpler to trust your ear. A simple test to see which is proper is to replace who/whom with "he/him." If "he" sounds right, use who; if "him" is right, use whom.

A number of authorities have abandoned the use of whom everywhere except following a preposition (To whom is it addressed?).

Who is your brother? You're looking at whom?

Rather than talking about essential or non-essential (non-restrictive) clauses, just remember that which clauses are always set off -- usually by a comma, but sometimes by a dash or with parentheses. So your choice always is between that and comma-which. If the comma seems out of place, use that. If you can tell which thing is being discussed without the which or that clause, use which.

Hint: Whenever you write which, try substituting that. If it fits, that
probably is the better choice. Another hint: The which adds a useful, but not necessary, piece of information. Imagine "by the way" following every which.

The bridge that spans the Connecticut River, which flows into Long Island Sound, is falling down.

Its is the possessive form of it -- one of many exceptions to the rule of apostrophes for possessives -- whereas it's is a contraction of it is or it has. The mistake of adding an apostrophe to the possessive pronoun its is perhaps the most frequent mistake among otherwise literate writers.

Its without an apostrophe means "belonging to it." It's with an apostrophe means "it is" or "it has." There is absolutely, positively, no such word as its'.

The trick: If you can replace its/it's in your sentence with "it is" or "it
has," then use it's; otherwise use its. Its is the neuter version of his or
her. Try plugging "his" or "her" into your sentence where you think it belongs. If it still works as a sentence grammatically (if not logically) then use its.

It's not easy to put the apostrophe in its place.

Farther applies to physical distance, further to metaphorical distance; further also means additional. You travel farther but pursue a topic further. In general, you can use farther for ideas of physical distance and further for everything else.

I can run farther than you, but let's discuss that further after the race.

Affect usually is a verb meaning to influence or to cause change in; effect usually is a noun meaning the result or outcome. When you affect something, you have an effect on it. Chances are that if the sentence calls for a verb, affect is the word you want; if the sentence calls for a noun, effect is the appropriate word. Affect as a noun is possible but best avoided unless you really know what you're doing.

What effect does this have on you? How does it affect you?