After the modal auxiliary βcouldβ, we use the base form (Vβ) of the verb.
π could + Vβ β could work
working on weekends β β could is not followed directly by V-ing.
worked on weekends β β could takes the base form, not the past form.
work on weekends β
β correct base form after could.
works on weekends β β works is a third-person singular form and cannot follow could.
Rule:
Modal auxiliary + Vβ
Examples: