Even the best writers occasionally insert errors or confuse two similar words. Online grammar checkers make it easier for you to verify that your sentences are well-formed and your grammar is excellent. There's no completely foolproof way to check your grammar online, though, so you'll still need to do some old-fashioned proofreading and editing.
Several free services will review your work for common misspellings and grammatical errors. GrammarBase, for example, will check for spelling and grammatical errors by reviewing sentence context, and it will also review sentence structure to ensure that your sentences are properly worded. Paper Rater offers similar services and also provides word suggestions and a review of the overall quality and tone of your paper.
Search engines such as Google review search requests for common errors, and they provide one simple way to detect typos and problems with word usage. Try plugging individual sentences into a search engine and see if it turns up any alternative results. For example, if you plug in, "How much cost do tables?" you might get a suggestion to replace your sentence with, "How much do tables cost?"
Online plagiarism checkers play a key role in ensuring paper quality. Particularly if you're paraphrasing a reference or citing research, it's easy to inadvertently plagiarize by using a similar sentence structure to that of the original source. Both GrammarBase and Paper Checker check for plagiarism, but Dustball's plagiarism checker provides a more detailed search. Dustball will return links to similarly worded websites and papers so that you can easily review them and see how to best revise your sentences.
No grammar checker is perfect, and checking sentence structure and grammar is generally more challenging than checking spelling. You'll need to closely read your paper to check for errors and improve quality. For example, you might not have any specific errors, but your sentence structure could be weak. A sentence such as "She gave it to me" might be more clearly written as "She gave me the box" if you haven't mentioned the box in the last few sentences.