Quite often, most of my proofreading and editing deals with basic errors of English.  Below are some of the most common errors:-

There, they're, their

There is a place; they're is a contraction of "they are"; and their is possessive, it means something which belongs to them.  Hence "there is the supermarket; they're going to get their dinner".

Where, were, we're

Where is a place; were is the plural past tense of "are"; we're is a contraction of "we are".  For example "where is the supermarket?  We're meeting someone there; we were so busy talking we missed the turning." 

Would have, should have, could have

NEVER "would of", "could of" or "should of".  The confusion arises when you say them quickly - "would've" sounds like "would of", but really isn't!

Whose and who's

Whose is the possessive pronoun, meaning something belonging to something or someone.  Who's is a contraction of "who is", "who was" or "who has".  You'd ask "whose book is this?" or "whose turn is it to wash up?"; you also ask "who's going to the shop" and say "anybody who's having trouble with their writing can use VJP Services" !

If in doubt, try replacing the word with "who is" or "who was".  In the examples above, "who is going to the shop" is obviously right, but "who is book is this" is not.

Regard and respect

It's "with regard to" and "in respect of", never "with respect to" or "in regards of".

Etc.

This is short for the latin "et cetera" and is always written "etc." - and because it's an abbreviation it should have a full stop after it, even in the middle of a sentence.  This doesn't mean that you need a capital letter to start the word after it; it is acceptable to write, scribble, scribe etc. and carry on the sentence with no break.

Plurals (or "the grocer's apostrophe")

When you're making a word into a plural, you don't need an apostrophe.  It's "the dogs are asleep", not "the dog's are asleep".  A tomato becomes tomatoes; a cat becomes cats; a woman becomes women.