10 rules to take into account when writing

1. To join two independent clauses, use a comma followed by a conjunction, a semicolon alone, or a semicolon followed by a sentence modifier.
2. Use commas to bracket nonrestrictive phrases, which are not essential to the sentence's meaning.
3. Do not use commas to bracket phrases that are essential to a sentence's meaning.
4. When beginning a sentence with an introductory phrase or an introductory (dependent) clause, include a comma.
5. To indicate possession, end a singular noun with an apostrophe followed by an "s". Otherwise, the noun's form seems plural.
6. Use proper punctuation to integrate a quotation into a sentence. If the introductory material is an independent clause, add the quotation after a colon. If the introductory material ends in "thinks," "saying," or some other verb indicating expression, use a comma.
7. Make the subject and verb agree with each other, not with a word that comes between them.
8. Be sure that a pronoun, a participial phrase, or an appositive refers clearly to the proper subject.
9. Use parallel construction to make a strong point and create a smooth flow.
10. Use the active voice unless you specifically need to use the passive.
11. Omit unnecessary words.

Source: Key English Writing (Longman - Pearson Mailisting)