Dictation is one of the best exercises that you can do to improve your grammar and vocabulary in French. It is an exercise that is done in France from primary school to middle school.
The exercise consists of listening to someone reading a French text and you must write what is read without looking at the text yourself. And then look at the text and correct any mistake that you have made. It may sound easy, but it is one the most difficult exercise. I’ve seen my students making mistakes to some most of the easiest sentences. Most of students in France are making a lot of mistakes as well.
This is because it requires your full attention on what is being read, therefore training your listening skills. But you must also have a good vocabulary, grammar and intuition of the French language in order to avoid mistakes when you are writing. A simple example would be that one forgets the “s” when the noun is put in plural.
It is very efficient and one of the quickest way to improve your French skills. You can do it at home, listening to video or audio online and start writing without looking at any text.