IELTS isn't simply a test of how much 'difficult' vocabulary and grammar you can use. It also tests your ability to do 'easy' things with the language, like making a simple sentence that contains no mistakes.
Speaking part 1 is a good example of this. In this part of the speaking test, you will impress the examiner by answering without hesitation, by speaking in a natural and fluid way, and by using error-free simple sentences (just look at my answers in yesterday's lesson to see what I mean). You can show off your best language later in the test, but you should start, in part 1, by showing the examiner that you can do the 'easy' things well.
Note: many people don't do the easy things well.