Most teachers use a correction code when they mark written work so that the student can do some self-correction. You can devise your own symbols but this table shows some possibilities.

Mark Error Indicated
/\ A word is missing
/ Start a new sentence
// Start a new paragraph
Gr Grammar error
Sp Spelling error
P Punctuation error
Art Error with articles (a, an, the)
c/unc Countable/uncountable error (you can use a/an before countable nouns but never before uncountable noun)
Wo Wrong word order
Ww Wrong word
Wt Wrong tense
Wf Wrong form
Irreg Irregular verb
? Unclear