How could I improve the register of my French language?

The register, or the formality of your language, is definitely something to consider both in written and spoken French. There are many ways to improve the register of the language, which will not only increase your vocabulary, but also your marks during assessments. For example, the repetition of 'mais' ('but' in French) is boring and unexciting. There are other options that you could employ, such as 'cependant', 'toutefois' or 'pourtant'. These all have the same meaning: 'however' or 'nevertheless'. Through using different connectives, you can vary your language and engage your peers or teacher more. A simple change from 'mais' to 'cependant' will certainly increase your marks too.