1.Every sentence must have a subject and a verb. An object is optional. Note that an imperative sentence may have a verb only, but the subject is understood.
2.When two singular subjects are connected by or, use a singular verb. The same is true for either/or and neither/nor.
3.In general, use the active voice (Cats eat fish) in preference to the passive voice (Fish are eaten by cats).
4.Adjectives usually come before a noun (except when a verb separates the adjective from the noun).
5.To show possession (who is the owner of something) use an apostrophe + s for singular owners, and s + apostrophe for plural owners.
6.Use the indefinite article a/an for countable nouns in general. Use the definite article the for specific countable nouns and all uncountable nouns.
7.The subject and verb must agree in number, that is a singular subject needs a singular verb and a plural subject needs a plural verb.
8.Spell a proper noun with an initial capital letter. A proper noun is a "name" of something, for example Josef, Mary, Russia, China, British Broadcasting Corporation, English.
9.Spell proper adjectives with an initial capital letter. Proper adjectives are made from proper nouns, for example Germany → German, Orwell → Orwellian, Machiavelli → Machiavellian.
10.The contraction he's can mean he is OR he has. Similarly, she's can mean she is OR she has, and it's can mean it is OR it has, and John's can mean John is OR John has.
11.The contraction he'd can mean he had OR he would. Similarly, they'd can mean they had OR they would.
12.Treat collective nouns (e.g. committee, company, board of directors) as singular OR plural. In BrE a collective noun is usually treated as plural, needing a plural verb and pronoun. In AmE a collective noun is often treated as singular, needing a singular verb and pronoun.