He worked on my car=he attempted to fix/improve my car. In this seminar, the work on writing aims to improve clarity, while the work on speech aims to improve pronunciation.= In this seminar, when we focus on your writing, we will attempt to improve its clarity. When we focus on your speech, we will attempt to improve your pronunciation. the work on writing= the effort and time that we will use to try to improve your writing.

Here work doesn't refer to something you make, rather it's the effort put in.. 比如说 “花了好大工夫” = put in a lot of work. 这里 “工夫” 和 “work” 很类似