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Canvas allows you to create an assignment for just a select few students without it affecting the grades of other students. This is perfect for an intensification of a course for a student or for when you have a special project for just one student or a few students, as well as cases when you want to set a different due date for a student requiring accommodations.
You begin by creating the assignment as you normally would in Canvas. When you get to the Assign to area where the due date is at the bottom of the assignment, however, you will want to remove the default EVERYONE setting and change it to just the student(s) it is being assigned to. Detailed instructions are here: How do I assign an assignment to an individual student?
As that page explains, only the student(s) you select will be able to be graded in the gradebook and the other students in the course will not even be aware that the separate assignment exists.
You can also use this method when you want to set different due dates within your class for a specific set of students. You do this by clicking the +ADD button at the bottom of the new assignment page and add the student name(s) as outlined above. Using this method, the default of EVERYONE for the class will change to EVERYONE ELSE so that it excludes the other student(s) you designated.