The section below is taken from The ABC Checklist for New Writers. It covers some of the grammar and punctuation areas new writers find difficult. It should be noted that this was written with the UK market in mind.

Apostrophes – Misused and missing
If you are one of those writers who have trouble with the dreaded apostrophe, you can get to grips with it by following a few basic rules. The apostrophe has two functions – to show possession and to replace a missing letter.

To show possession:
An apostrophe is added before the letter s to denote possession.
My mother’s garden: The garden that belongs to my mother.
Jane’s car: The car that belongs to Jane.
The doctor’s surgery: The surgery that belongs to the doctor.
The teacher’s books: The books that belong to the teacher.

But, if you are writing about more than one doctor or teacher, the apostrophe is added after the plural.
The doctors’ surgery: The surgery that belongs to the doctors.
The teachers’ books: The books that belong to the teachers.

When a word already ends in s, a similar rule applies. The apostrophe is placed after the s with no extra s needed.
Jess’ shoes
Mr Jones’ house
The business’ phone number

An apostrophe is not used when indicating a straightforward plural.
Latest movies – not latest movie’s
Special offers – not special offer’s

To replace a missing letter:
An apostrophe can be used to indicate where a letter has been missed out of a word.
Isn’t – is not
Wasn’t – was not
Don’t – do not
We’re – we are
These contractions are often used in speech, or when you want to write in a less formal style.

It’s/its:
The rule that trips up many people is the one that applies to the words it’s and its. It’s is the abbreviated form of it is and the apostrophe is used in place of the omitted letter ‘i’ in the word is.
It’s a beautiful day: It is a beautiful day.
This shows it’s abbreviated: This shows it is abbreviated.
Remember, the apostrophe is only used here to replace a missing letter.

But an apostrophe is not used when writing about something that belongs to ‘it’.
The lorry shed its load
The dog is asleep in its basket

Spelling & Grammar – Common Errors to Avoid
The list below covers some of the basic errors which signal amateur status to an editor.

Loose/lose: If the collar is loose (not tight enough) you might lose (mislay) the dog.

Lay/lie: Lay (put somewhere) those papers on the desk. I’m going to lie (in a horizontal position) down.

Then/than: He has more work than I have, but when I have finished writing, then I can help him.

Who/whom:
Who replaces the subject of the sentence. He is missing the point – who is missing the point? He is. Whom replaces the object; this means whom replaces him or her.
You also use whom when it follows a preposition, such as: for, to, or with.
For whom; for him.
To whom; to her.
With whom; with them.

Who’s/whose: Who’s (who is, the apostrophe replaces the missing ‘i’) going to take the books? Whose (possessive) books are they?

I/me: The way to decide which of these to use is to take the other person out of the sentence and see which makes sense. (Maureen and) I write for a living. The contract was written for (Maureen and) me.

Less/fewer: An easy way to remember this one is: if you can count it, or divide it, you use fewer, otherwise you should use less. There was less sugar in the bowl (you cannot count sugar). There were fewer cubes of sugar in the bowl (you can count the cubes of sugar).

As/like: Like is used before a noun – They looked like giants. As is used before a verb, or a clause containing a verb – As I feared, the weather was awful.

There/they’re/their: There is a way to remember the correct use of these. They’re (they are, the apostrophe replaces the missing ‘a’) easy to get right if we think about their (possessive) uses.

Your/you’re: As with other examples, one is a possessive and the other a contraction – it is your (possessive) duty to learn the difference; you’re (you are – missing ‘a’ replaced by apostrophe) going to find it easy.

Lets/let’s: The word lets (without an apostrophe) has many uses: Mary lets John stay. The landlord lets the apartment. Let’s, with an apostrophe, has only one use – it means let us and the apostrophe replaces the missing ‘u’.

Welfare/farewell: When you are concerned with someone’s welfare, only one ‘l’ is required, but, when saying farewell to someone, the ‘l’ doubles up – imagine both hands waving.

To/two/too: Two is a number – two cars, two people, or two books.
Working out the difference between to and too might be harder. Perhaps the easiest way to remember it is too means ‘as well’. I’m going, too – this means I’m going as well (the too always needs a comma before it). I’m going to (go somewhere or do something) the park or the shop (both nouns) or to work, to run, to write (verbs).

Here/hear: Here is the shop; here is the dog; here is the reason. Here is the opposite of there. Hear is a verb. We hear someone speaking; we hear rumours. Members of parliament shout: hear, hear – meaning I have heard and I agree.

Practice/practise: Practice is a noun and practise is a verb. John practises medicine at his practice. A friend of ours always remembers the rule by reminding herself that ice is a noun – so practice (ice) is also a noun.

Of/have: Never use of when you mean have. She should have gone to work. He could have left early. They would have preferred to holiday abroad. If these are contracted they become: She should’ve gone to work. He could’ve left early. They would’ve preferred to holiday abroad.

Never, never, never use: should of, could of, or would of.
Flash 500 Competition for flash fiction up to 500 words and Flash 500 Humour Verse Competition for amusing poems up to 30 lines

Lorraine, you may remember Ronald Ridout from schooldays. Even in 1974 he wrote,
"When a word already ends in s and, in making it possessive, we give it another syllable, it is now customary to add an s as well as the apostrophe, e.g.:
Wrong: Jones' idea is a good one.
Right: Jones's idea is a good one."
AG AG Home Page

As I understand it - and these are my terms, not technical ones - "which" begins a qualifying statement, whereas "that" is a differentiating tool. So:

"The book, which had a green cover, always went for its afternoon walk at 2pm." Here we are just elaborating on the kind of book it is.

"The book that Jones liked to read before before supper always went for..." Here, on the other hand, we are trying to distinguish one particular book, by differentiating it from all the other books in Jones' library.

So, finally, if we said "The book that had a green cover always went..." we would be saying something different from (and PLEASE don't tell me that saying "different to" is acceptable, whatever Fowler says; and say you all agree that "different than" shoudl be up with treason on the high seas as a capital offence) the first statement.

As a rule of thumb, "that" is correct wherever your sentence provides an answer ("the x that...") to an implied question "Which x?" Note also that one uses commas differently in each case.

Is that about right, Lorraine?

And by the way, on the subject of apostrophes, the usage I didn't see mentioned was the ubiquitous and awful CD's, DVD's and the like - this is wrong for the reasons given in Lorraine's piece - an apostrophe replaces a missing letter/s. So if we were using apostrophes we would say C'D's or D'V'D's - having only one makes no sense at all. In practice, we treat acronyms like this as words and just use "s".

And on the subject, full stops work in the same way - so Prof. would be correct because the stop shows that there are missing letters. Dr. and Mr./Mrs. are not correct because the missing letters are not where the stop is. Songs from the Other Side of the Wall is just £0.70 on Kindle

I almost didn't put that part in because it is really, as the excellent Lynne Truss says, a matter of style and preference that is definitely not set in stone (page 56 of Eats, Shoots and Leaves). When I did my research for this I found a greater proportion of reference books going without the second s. Flash 500 Competition for flash fiction up to 500 words and Flash 500 Humour Verse Competition for amusing poems up to 30 lines

I don't know, Tommi. Must be an endorsement of one of them. Personally I use both depending on the mood I'm in. Not at the same time of course. Although that would cover the options. Sometimes I'm keeping up with the Jones's and sometimes not. AG. AG Home Page