Mistakes in these areas generally won't stop the reader understanding your points, but they can make your ideas less clear. These are things that you can focus on when you are proofreading your work.
countable: singular and plural forms (apple | apples)
uncountable: no plural form (water)
explanation and exercises (BC)
exercises (Ox)
Words and phrases that show how much, or how many.
basic exercises (Ox)
advanced exercises (Ox)
order the sentences (NEF)
Punctuation is important because it makes written language easier to understand.
general punctuation (EF)
commas: explanation & examples (EF)
full stops: explanation & examples (EF)
capital letters & apostrophes: explanation & exercises (BC)