Good English

Sentence formation

All sentences should contain a verb. Some contain only a verb e.g. Go! or Help! Most contain a verb, subject and object e.g. The cat sat on the mat. Sub-clauses may be added, with the aid of a conjunction, as in this present sentence. The most common problem is running sentences together with just commas between them. Closely linked sentences can be separated by a semicolon.

cross Mary had a little lamb, its fleece was white as snow.
tick Mary had a little lamb; its fleece was white as snow.
tick Mary had a little lamb. Its fleece was white as snow.

Initial capitals

Use initial capitals only for the first word of a sentence, titles and proper nouns, NOT for generic nouns, EXCEPT when they become a proper name or title (or part of one).

cross Kitchen; Bedroom; First Floor; he is a Farmer/Architect/Surveyor; a Manor House; a Mill; the building faces South; the Eighteenth Century; Medieval.
tickkitchen; bedroom; first floor; farmer; architect; surveyor; manor house; mill; stone; street; road; crescent; terrace; the building faces south; the eighteenth century; medieval; the church in the village.
tick The Crimson Salon; The Painted Chamber; John Ruskin, The Stones of Venice; The County Surveyor; The Royal Institute of British Architects; Bindon Manor; Otterton Mill; Bath Stone; Flemish Bond; Southside Street; Mafeking Terrace; the Westcountry; our branch in the North; Georgian; Victorian; St Mark's Church; the Church of England; the established Church.

The use of the apostrophe

The apostrophe indicates either: a contraction (can't = cannot) or the possessive (Jane's computer). It should NOT be added at random to words or dates ending in s.
cross 1950's.
While most possessives need an apostrophe, his, hers, its, your, yours and whose do not.

it's = it is its = belonging to it
you're = you are your, yours = belonging to you
who's = who is whose = belonging to whom

Other homophones (words that sound the same, but have different meanings)

bare = naked bear = carry, support, endure
cite = quote, refer to site = locate, location sight = vision
complementary = completing complimentary = flattering; free of charge
dew = moisture from condensation of water vapour due = owing, fair, fitting
discreet = prudent, circumspect discrete = separate
flair = natural aptitude flare = blaze of light, burst of anger
formally = in a formal manner formerly = previously
grate = cast-iron fire bars and frame of fireplace; scrape into shreds great = large, important
hear = perceive by ear; listen to here = in this place
peace = tranquility piece = portion, part
pleural = related to the membrane covering the surface of the lung plural = more than one
principal = chief principle = moral rule
residence = house, place of abode residents = inhabitants
role = function, part roll = revolve, coil up, flow in waves
shear = cut or clip off sheer = complete, unmixed, transparent
sort = class, order, type, separate into classes sought = looked for
stationary = not moving stationery = writing materials
stile = steps over a hedge or fence style = fashion, manner, type
there = in or towards that place; at that point or stage their= belonging to them
troop = company, band troupe = group of performers
wares = merchandise wears = is clothed in; diminishes by use

Misused words

The majority = the greater number. Not a synonym for the major part, or most.
Unique = the only one (from Latin unus = one). Not a synonym for unusual. So things cannot be 'very unique' or 'slightly unique'.

Them use of the hyphen

Composite adjectives should be linked by a hyphen, to avoid possible confusions. Example: 'the little used car'. Is this a small, second-hand car? Or a car infrequently used? 'The little-used car' makes the matter plain.
tickIn the eighteenth century the fashion switched to doors with six panels. Those are eighteenth-century six-panel doors.

For a more detailed guide see Common Errors in English by Paul Brians of Washington State University