Using apostrophes ( ' )

Short forms and pronouns
Short forms like «I'm» and «don't» should only be used in informal, conversational writing and when reporting speech. Although it is correct to use an apostrophe to indicate a missing letter and write: «aren't», can't», «isn't», «it's» etc., short forms are to be avoided in official letters, reports, academic papers or theses and other types of formal English. Here, the expected forms are: «are not», «cannot» (one word), «is not», «it is». Using short forms wrongly not only looks sloppy, it also leads to mistakes like confusing «it's» with the possessive pronoun «its», which both sound the same.

Pronouns like: «yours», «his/hers», «its», «our/ours» and «their/theirs» never take an apostrophe in the genitive. Use the «of-form» instead, as in: «that husband of yours». A few pronouns that take an apostrophe in the genitive combine «some», «any», «every» and «no» with « -body», «-one» and «-other», as in: «Someone's life is at stake». Also, «us» is sometimes contracted with verbs like «let», as in «Let's go».

Dates and abbreviations
A useful rule is to use 's to form genitives (90's and IBM's), and a small «s» to form plurals (90s and PCs). Thus the «1990's problems» mean: «the problems of the 1990s». Similarly, «the PC's future» means the «the future of PCs». Nevertheless, it is recommended to use 's with single letters (such as in a formula) that might be confusing to read. Example: «Dot your i's and cross your t's» (tidy up what you have written).

Nouns
Apostrophes are used to form the genitive when they refer to people and animals, as well as things people are fond of. Two general rules are:

1. Place 's after singular nouns that end in «s», or after a noun (singular or plural) that does not end in «s». (Examples: «Charles's», «the class's teacher», «dog's dinner», «a child's toys», «the children's toys», «the boat's performance».) An exception is names of people ending in «s» that sometimes only have an apostrophe: «King James' Bible».  2. Place an apostrophe after plural nouns that end in «s». (Examples: «the classes' teacher» and «the dogs' dinner».) These rules make it clear whether a class has one teacher (class's) or shares a teacher (classes'), or whether Fido has to share his dinner with others.

Tricky words

Acoustic, acoustical, acoustics

Acoustic (adj.), (Norw. akustisk, høre-, lyd-) This refers to hearing, sound or the science of sound. In anatomy, we have the «acoustic nerve» (Norw. hørenerve), and in sound, an «acoustic image» (Norw. lydbilde). «Acoustic» is more frequent than «acoustical». Examples: «acoustic feedback» and «acoustic guitar».

Acoustical (adj.), (Norw. akustisk). «Acoustical» is found in terms like: «acoustical measurement» (Norw. akustisk måling) and «acoustical shadow» (Norw. skyggesone). There is very little difference between «acoustic» and «acoustical» but they are rarely interchanged.

Acoustics (noun), (Norw. akustikk). This has two meanings. One is the science of sound, when it always takes a singular verb: «Acoustics is a challenging field». In the second sense, it means the acoustic properties of a room (Norw. romakustikk) or a building. Here, it takes a plural verb: «the acoustics in the Greek amphitheatre are perfect».

Bath, bathtub, bathe
bath, bathtub (noun), (Norw. badekar). There is a transatlantic difference here. In BE, you take a bath, in a bath. In AE, the object you sit in is usually called a bathtub or tub.

to take a bath is also a slang expression meaning suffer a large financial loss, as in: «We took a bath in Hong Kong last year».

bathe verb (Norw. bade). In BE usage, if you «bathe» or «swim» you do so in a pool, river, lake or in the sea. In AE, «bathe» and «swim» are used as in Britain, but «bathe» also means washing yourself in the tub. Thus: «I am going upstairs to bathe» may sound rather unusual in Britain. «Bathe» is also used to wash something carefully in both BE and AE: «bathe a wound» or «bathe your eyes».

Enlightening English

Perhaps the «bath - bathe» problems are the cause of this gem on a Japanese hotel room notice:
«Please to bathe within the tub».

Another intriguing hotel notice, this time from Yugoslavia:
«The flattening of underwear with pleasure is the job of the chambermaid».

Finally, a charming notice outside a Hong Kong clothes shop:
«Ladies can have a fit upstairs».