Note, the following all apply to British English, other versions of English often have different rules.
There is never an apostrophe in ‘theirs’ (you can’t write ‘their’s’ or ‘theirs”). ‘Theirs’ is already possessive.
Generic nouns (street, river, lake) are never capitalised.
But when part of a proper noun (a place name) they are always capitalised, so: ‘Dali Old Town’ (not ‘Dali old town’), ‘Erhai Lake’ not ‘Erhai lake’, ‘Yunnan Province’ etc.
When a geographical feature comes before the proper name, it is nearly always part of the name so should be capitalised: ‘River Thames’, not ‘river Thames’. However in a list of names it can be used as a generic noun, therefore not capitalised, as in: ‘the villages of Haiyin and Nuogan’ (not ‘the Villages of Haiyin and Nuogan’).
All religions are proper nouns (Christianity), and are also capitalised when used as an adjective; so ‘Muslim’, ‘Buddhist’ etc.
Species names in English are lowercase, as in oak or leopard, unless the name is derived from the name of a person or place, as in Armand pine or Siberian tiger. The names of cultivars are always capitalised, as in Empire apple, Red Delicious apple etc. In scientific names, the genus is capitalised but the second part (the species name in Latin) is not, even when derived from the name of a place or person. So the tea plant is Camellia sinensis, the giant panda is Ailuropoda melanoleuca.
‘BC’ (Before Christ) comes after the date, so 200 BC. ‘AD’ (from the Latin phrase ‘Anno Domini’ meaning ‘In the year of our Lord’) traditionally came before the year, as in AD 200 (‘In the year 200 of our Lord’), though for a long time it has become increasingly common to put it after (which translates awkwardly as ‘200 in the year’).
‘etc.’ is always followed by a full stop, unless it comes at the end of a sentence. You never write ‘etc..’.
Initials in peoples’ names have a point but no space, but there’s a space between the initials and the name: F.W. de Klerk.
For compass directions, it is acceptable to have a hyphen (‘north-west’) or one word (‘northwest’), both are fine. It is important to be consistent in a single piece of writing however (I use ‘northwest’, no hyphen).
In British (Australian, NZ) English, put direct speech within single quotation marks (‘inverted commas’), and use double quotation marks for direct speech quoted within direct speech:
‘He told me to “mind my own business”.’
In American (and Canadian) English it is the opposite:
“He told me to ‘mind my own business’.”
BUT in UK magazines and newspapers the American use (double > single quotation marks) is now standard.
Using quotation marks for book titles (single in the UK, double in America) is now considered old fashioned.
Is used for all abbreviations: GNP, A-levels, AD, BC,PhD
But NOT used for roman numerals, chemical elements or temperature (Celsius, Fahrenheit).