As a quick example, the link grammar immediately highlighted that your second sentence: > I have been living in London for past 6 years > and my English st

As a quick example, the link grammar immediately highlighted that your second sentence: > I have been living in London for past 6 years > and my English still sucks. doesn't parse. You need a "the" before "past" for natural sounding English. It really might help a lot, and perhaps get your error rate down to an acceptable level without having to pay for external assistance. As a footnote, in the fourth sentence it did not pick up that you meant "conversation" and not "conversion."