Some IELTS students spend a lot of time preparing "difficult" sentence structures to impress the examiner. One particular issue that people ask me about is the use of 'compound' and 'complex' sentences.
The first thing I would say about compound and complex sentences is that they are not "difficult". Here in the UK, children learn about these sentence structures in primary school (click here to see how children are taught to understand sentence types). If you use words like and, but, because, although, while, when to connect ideas, you will automatically write a longer sentence that will be either compound or complex.
The second thing I would say is that you should not be worrying about these things when you are writing an IELTS essay. Just focus on answering the question and explaining your ideas well. I'm sure you'll naturally use "connecting" words to join ideas and create some longer sentences, so there's no need to worry about this at all.