I have finished all the VP reading section books and questions. Is it helpful to re-read those articles again or i should find a new articles? because usually i need to spend 4~6 mins to read an new articles on gmat reading part. But when i read those i have read before i only need like 1-2 mins to do so.

How can I improve sentence correction on gmat? because im lake of grammar knowledge so the VP books explanations are not enough for me and in class almost everybody is English speaker so instructor just briefly go through sentence correction part.

thanks

I totally feel you about the not totally native English speaker.

I have found this free vocabulary words builder tool (http://www.vocabulary-words.com/) that lets me mark words for review, review them, quiz myself etc.

In addition, I recommend you try to pay more attention to exactly what type of question you are getting wrong (like content sub category).