I sometimes read student assignments where the grammar and writing style is poor. Paragraphs don't really flow. Sentences go on for ever. The ordering of words is awkward. Instead of concisely introducing the subject of the sentence, it goes on forever.
I'd like to be able to recommend resources to such students. Ideally, such a resources would be available online so I could just give them a link or two to some writing style and grammar materials with exercises.
What is a good resource (preferably online) for teaching English speaking university students how to improve their writing style?