If vocabulary represents the bricks that make up a house, then grammar is the cement that glues it all together. It can sometimes be difficult but understanding how to put sentences together and how they work, helps massively when learning a language. Take the time to invest in your grammar ability and it will pay off massively in the long run.
If you aren’t sure what your level is, then take this English Grammar Placement Test first. It will tell you where your weaknesses and strengths are, so you know what you should study next.