Which and that

The traditional rule is that you use which to add extra information that is separated off from the main idea by commas and that for information essential to the meaning of the sentence.

Using grammar terminology, clauses that provide extra information are non-restrictive clauses (also known as non-defining clauses) and clauses that provide essential information are restrictive clauses (also known as defining clauses).

Compare the following sentences.

The bike that is in the garage needs fixing. (There may be other bikes on the property, but it’s the one in the garage that needs fixing.)
The bike, which is in the garage, needs fixing. (The bike needs fixing. By the way, it’s in the garage.)

However, which and that are now often used interchangeably for adding extra information.

Many of the rules which we take for granted are not foolproof.
Many of the rules that we take for granted are not foolproof.

Sometimes using the traditional rules adds clarity. The Australian Commonwealth Style manual gives this example.

The research findings that were likely to cause embarrassment were never circulated.
The research findings which were likely to cause embarrassment were never circulated.
The research findings, which were likely to cause embarrassment, were never circulated.

It says: ‘The first example makes it clear that the research findings not circulated were the ones likely to cause embarrassment. In the third example, it is plain that none of the recommendations was circulated. The situation described in the middle example is ambiguous: were all of the findings withheld or just the embarrassing ones?’

Who and whom

The rule is to use who for the subject of a sentence and whom for the object. (A subject performs the action and the object receives it.)

The man whom I met yesterday is a barrister. (Subject = I. The man = object)
The man who I met yesterday is a barrister. (This used to be considered incorrect, but is now commonly used.)

Tip: If you can replace the object with him (or her), use whom.

I met him yesterday. The man whom I met yesterday…

Whom appears to be vanishing in questions.

Whom do you admire most? (Subject = you, so this sentence is more ‘correct’)
Who do you admire most? (Commonly used)

Punctuation with however

An old-fashioned rule was that you shouldn’t start a sentence with however, but today however is often used at the beginning of a sentence to mean ‘but’, ‘nevertheless’ or ‘regardless of the fact’.

When you use however at the beginning of a sentence, use a comma after it.

However, the other alternative looks more attractive.

As well as indicating a pause, the comma distinguishes however meaning ‘in whatever way’.

However, you decide.
However you decide.
I want you to finish your report today however busy you are.

When you use however in the middle of a sentence to mean ‘nevertheless’, you need commas.

The budget was no fiscal revolution. It did, however, mark the first ‘real’ step towards tax reform.

Use semicolons to separate clauses
When you use however in the middle of a sentence to separate two clauses, it is usually separated with a semicolon and a comma (… ;however,…). Many modern writers use a comma instead of the semicolon, but the semicolon is still regarded as more correct.

The budget was no fiscal revolution; however, it did mark the first ‘real’ step towards tax reform.

The rationale for using a semicolon is that however is an adverbial conjunct (also known as a conjunctive adverb or conjunct). Unlike a conventional adverb, which affects the meaning of a single word or phrase, however affects the meaning of a clause.

You can avoid the semicolon if you replace however with but or yet.

The budget was no fiscal revolution, but it did mark the first ‘real’ step towards tax reform.

The subjunctive

One of the most common usages of the subjunctive is to express wishes or conditions contrary to fact. The only grammatical indication of this mood is that with singular subjects we use were instead of was.

If I were a carpenter, and you were a lady, would you marry me anyway?
If I were a rich man,…

Today, many people would use was instead.

If I was a carpenter, and you were a lady, would you marry me anyway?
If I was a rich man,…

Mandative subjunctive
The mandative subjunctive consists of the base form of the verb (e.g. love) and is used in demands, recommendations and suggestions. It is only apparent in the third person singular of the present tense.

I suggest that she attend the meeting.
I demand that he withdraw his application.