Editing is one of the most important parts of writing, but it's hard to know where to start sometimes. Hemingway is a web app that highlights hard-to-read sentences, adverbs, complicated phrases and passive voice so you can make your writing more clear.
The idea behind Hemingway is simple: it highlights anything that makes your text hard to read so you know what to fix. Like most syntax highlighting, the work of fixing it is up to you, but Hemingway makes it easier to spot the faults. It highlights complex sentences in red or yellow, depending on how complicated they are. Then, it highlights adverbs in blue, complex words in purple, and passive voice in green. As you go through and clean up you text, Hemingway keeps track of its readability and counts down the fixes.
You don't need to follow its suggestions in every case, but it's a good way to glance at the readability of a short block of text. If nothing else, Hemingway is great to check for clarity in something short like email.