Nyimalay, i think u shd need many practices. In my student life, i had done so many exercises for single pattern of basic grammer. After studying some amt of patterns, i had to do exercises on mix pattern of grammer. Do u hear that practice make perfect? After doing many exercises, u will know automatically what is wrong and what is right. It is very difficult to explain but it is real for studying English as a second language. U remember almost all the pattern of grammer. u wiil become expert after doing many, many exercise from various grammer text books, but two or three is seem to be enough. Don't be wavy. Emphasize on what shd u do. Ok.