A natural way that you can learn how the rules of writing should accommodate mathematical elements is by reading examples of writing featuring those elements, thus acclimating yourself to the normative style through exposure. To this end, here is an assortment of mathematical writing, most of it my own, that you can peruse.
Activities, Quizzes, and Exams
These samples are solutions/responses I wrote with my students in mind as my audience. That is, as models, these are more verbose, and more instructional, than what I would expect see a student write.
Notes, Articles, and Examples
Mathematical Writing Guides
For direct instruction on mathematical writing beyond just examples see my brief mathematical writing guide, or A Guide to Writing Mathematics by Kevin Lee.