The following are the most common corrections and comments that I make on student essays, with explanations:

A wavy underline shows that there is a problem with the passage, similar to "awkward," below.

A straight underline may mark a particularly important passage in the essay, or one on which I have written comments at the end of the essay or in the margin. In the case of titles, it indicates that underlining (or italics) should have been used.

Insert a word or letter. By itself, this indicates missing upper punctuation, such as an apostrophe or quotation mark.

Insert a word or letter. By itself, this indicates missing lower puctuation, such as a comma or period.

The
word or phrase circled has something wrong with it: spelling, tense,
case, number, and so on. Sometimes I will draw a line to another
circle, in order to show you what the problem is.

The letters or words or punctuation here should be in reverse order. See, for example, "split infinitive," below.

The
punctuation circled must be paired with identical punctuation where the
arrow points. That is, the punctuation (usually commas) should appear
at both places, or neither.

Omit
the letter, or letters, or space indicated. For full words and longer
passages, I may just draw a line through them, or circle them and write
"omit."

Insert a space here.

Omit the space here.
- ! An exclamation point indicates a surprising statement, which may be either a positive or negative reaction on my part.
- ? A question mark indicates that I am not sure what you mean. See "meaning," below.
- agreement: The subject and verb, or pronoun and antecedent, do not agree in number.
- awkward: Poorly written, though why it is so may not be clear to me, or may be too complicated to explain in a short written comment.
- dangling: An introductory participial or
prepositional phrase "dangles" when its understood subject is not the
subject of the sentence -- for example, "Having worked hard on the
greensheets, they were handed out in class."
- integration: Quotations must be integrated
into your prose both logically and grammatically, so that they read as
part of your essay. Incidentally, avoid long, indented quotations,
especially in essays as short as ones you will be writing for this
class.
- fragment: The sentence lacks a subject or verb. See "!" above for an example.
- meaning: The passage is not sufficiently clear (or not clear at all), either on its own or in context.
- parallelism: Two or more phrases in a sentence
which should be grammatically parallel are not -- for example, "To sin is
human, forgiving divine."
- run-on: A sentence that goes on too long,
usually in a grammatical sense. This includes the infamous "comma
splice," two independent clauses joined only by a comma.
- split infinitive: Split
infinitives are becoming increasingly accepted. I continue to mark them
to be sure you know that the infinitives are split, and that there may
be reason to avoid those constructions.
- subordination: A run-on sentence that could be made grammatical by subordinating the verb of one of the independent clauses.
- syntax: Something, usually grammatical, is wrong with the words or their order in the sentence.
- tense, tense shift: The verb tenses have shifted (for example, from past to present tense) for no apparent reason.