Conventions of Standard Written English
Find the skeleton, then judge
Most convention questions fail candidates not because the rule is unknown but because the sentence is long. Strip a sentence to subject + verb + object, decide, then re-read it in full.
Rules that recur: subject–verb agreement - the verb agrees with the true subject, not the nearest noun ('The box of old photographs was...'); indefinite pronouns (each, everyone, neither, none) take singular verbs in formal prose; 'the number' is singular, 'a number' is plural. Pronoun case and agreement - prepositions take object forms ('between you and me'); 'who' is subject, 'whom' object; singular indefinite antecedents take singular pronouns. Modification - an introductory participial phrase must describe the subject of the main clause ('Walking home, I saw...' not 'Walking home, the rain...'). Parallelism - items in a list, and both halves of 'not only... but also' or 'either... or', must match in form. Punctuation - a comma cannot join two independent clauses (comma splice); a semicolon can; a colon must follow a complete clause; apostrophes mark possession, not plurals ('the 1990s'; 'its' versus 'it's').
Traps: sentences that sound informal but are correct; a plural noun sitting right beside the verb; and a correct sentence offered alongside a smoother but ungrammatical one. Judge structure, not rhythm.
Rule of thumb: cover the interrupting phrase with your finger. If the verb no longer matches the subject you have found, it is wrong.
Worked example: 'The collection of rare coins, along with several letters, (was/were) donated.' Skeleton: 'The collection... was donated.' Singular.