You can get English sentences corrected online at PaperRater.com, GingerSoftware.com and SpellCheckPlus.com. Each website has a free basic grammar-checker, though PaperRater.com and GingerSoftware.com each offer more comprehensive grammar-checkers at paying membership levels.
PaperRater.com shares its grammar-checker with GingerSoftware.com, but the interface at PaperRater.com lets you enter additional information, such as works cited, references or a bibliography. You can paste or upload a text and share the education level of the author.
The sample version of Ginger's grammar-checker, at GingerSoftware.com, entails typing or pasting text into an empty field. Checking grammar one or two sentences at a time is best. Ginger has a tendency to miss random grammatical errors among many in longer paragraphs. Grammatical errors are highlighted in light blue.
At SpellCheckPlus.com, you type or paste text into an empty field. SpellCheckPlus.com simultaneously checks grammar and spelling. Errors are highlighted, and hovering over highlighted text reveals an explanation of the error and possible corrections.