I would still seek to answer the three main questions in the cover letter as well (Why commercial law, Why the firm, Why me) but I would seek to employ more of a 'narrative structure' and speak more about personal experiences. Essentially, you should describe your personal, academic, and professional trajectory more broadly, focusing on how your journey had led to where you are now. In the application form question, while answering the same questions, you should write in a to-the-point style and implement a very clear structure. Your points should be a lot more factual and the experiences you describe should be more academic/work related.
As for the 'Why the firm' section, as
@Jessica Booker mentioned, you can make the application form question answer more around the work opportunities (seats and rotations; secondments; practice area and sector expertise; client base etc) and the cover letter answer more around the working environment and why that is a great fit for your personality (here, you can discuss the training programme specifics and culture more, but perhaps also the working environment in the teams/practice areas of the firm you are interested in and more broadly the working environment at a type of commercial law firm like
Taylor Wessing).
The one thing I will add is that while avoiding repeating the experiences you mention is ideal, it is not an issue if the points that you are illustrating through those experiences and the details of the experiences you are describing are different in the application form answer and the cover letter. As said before, the cover letter is meant to allow you to show your personality more. As such, you could also provide a more detailed explanation as to your fundamental motivations from a personal perspective in it. For instance, say your basic answer to 'Why commercial law' in the application form question looks something like this:
- Becoming a commercial solicitor because this is the only career path that involves working at the junction point of legal and business advisory.
- I have this academic/professional experience (explained with a STAR structure) which has shown me I am attracted to/a good fit for a career that involves both business and legal advisory.
- Therefore, I want to be a commercial solicitor.
In the cover letter, you could discuss the same experience and underlying motivation for commercial law, but from a different perspective. You could use the opportunity to dig a lot deeper into your deeper personal interests and motivations: what is it about you that makes you attracted to an advisory role as opposed to an executive one? What is it about the legal and business world that would make learning about it more interesting than other fields? When did you realise you had these interests, and did you consider any other career paths? If so, which did you consider, and what steps did you take to confirm that commercial law is indeed the optimal choice? How did this impact you and lead to the evolution of your professional interests and preferences over time? The point is, you can delve a lot deeper into all the thought processes and less significant prior experiences that led up to the point at which you are now. While the final conclusion of this exposition might be the same attraction to business and legal advisory you mention in your application form answer, this does add a lot of value to the recruiter's understanding of you as a person. This is a lot of useful information and contextualization that would simply be impossible to include in the application form answer due to the space constraints and would therefore likely not be deemed a mere repetition.