i was fortunate enough to have a hard-ass english teacher (had been teaching for over 50 years) for 1 year of highschool that made us all use Elements of Style. we had a daily 1 page writing assignment where you got an F if he found more than 2 grammar mistakes, regardless of content. hated it then, happy now that i have decent grammar (not on web forums though :o).
just to elaborate on greg's post above, here are the common verb tenses and example sentences:
I study English everyday.
Two years ago, I studied English in England.
If you are having problems, I will help you study English.
I am going to study English next year.
I am studying English now.
I was studying English when you called yesterday.
I will be studying English when you arrive tonight.
I am going to be studying English when you arrive tonight.
I have studied English in several different countries.
I had studied a little English before I moved to the U.S.
I will have studied every tense by the time I finish this course.
I am going to have studied every tense by the time I finish this course.
Present Perfect Continuous
I have been studying English for five years.
Past Perfect Continuous
I had been studying English for five years before I moved to the U.S.
Future Perfect Continuous
I will have been studying English for over two hours by the time you arrive.
I am going to have been studying English for over two hours by the time you arrive.