I have been an instructor for communication and English Language for US, Canadian, Indian and Middle East geographies. Throughout this experience there was one common thing I noticed, be it a native English speaker or a non-native one, both lack their hold over grammar. While speaking or writing, what we need to keep in mind is to convey maximum through minimal words. In doing so grammatical errors should be completely avoided. Thus, I have always made it a point to touch the roots (basics) first and then slowly move towards the advanced level. Although the students may demand a higher level, learning the English language doesn't entail completing levels. It comprises speaking or writing error-free sentences. English is like Math, you need to have thorough practice to master the art and once you do that there is no going back.