Agreement: "The causes of global warming have been determined."
Tense: "Prior to the conference, the paper will have been completely revised."
Possessives: "The participants' opinions were valued."
Plurals: "The seven types of oxen were included."
of, with, to, from, for, etc.
a, the (missing or superfluous)
its, I/me, his/him, her/hers
Commas (in series, to set off non-restrictive clauses, in compound sentences)
Colons (to introduce list; to expand or explain)
Semicolons (to form parallel structure; not to introduce lists)
Periods (after citations in APA style; in abbreviations)
Hyphenation (compound adjectives, compound words)
Here's a sample of an essay that needed only basic proofreading.
There are a few wording errors, a run-on sentence is corrected, a hyphen is added, and a missing space is inserted. Note that although only a few changes are required, every word and every sentence has been carefully reviewed.