Resources for English speaking students with poor command of grammar and writing style
I know that this can be the source of great frustration for teachers, especially when they do not teach writing. However, writing is one of those things, like other communication skills, that I think we all need to teach (at least a little bit) if for no other reason that our students will fail if they cannot compose a well-reasoned essay, even if they understand the material from our subject quite well.

There are plenty of universities which provide very good source material for this. Here is one from the US. Here is one from the UK. I have used materials from both (and many others) but I do not find that I can simply point the students to these sites and expect everything to work out.

As educators, sometimes we need to teach things which are more fundamental in nature. Unless your school has a 'writing center' so you can direct students there for help (my school does not). I find I need to spend 1-2 hours per semester to cover core writing issues.