In a chat the other day, one of my fellow teachers expressed her regret about some of our non-English major students not being able to write grammatically correct sentences even after having learned English for so long. They all concurred with my remarks over students' inability to write well related directly to their reluctance to read extensively.
Students can't write well because they don't read much. This is simple physics. Writing is an act to output what has been input. It's impossible for studenst to produce anything if they don't first plant seeds.
From my experience, even English major students mistakenly believe that attending writing classes alone is enough to learn to write well. Yes, writing skills are taught in writing classes, however, if students do not first read, store, and absorb adequate volume of vocabulary and observe numbers of sentence patterns usages, a decent paragraph or a readable essay is hardly to be expected.
To write well or to speak or listen better, one must first read. That's it!