We usually give teaching assistants the ta (i.e. 5) permission level.
If you go to Course Configuration and look at Permissions you will see the default permission levels. TA's can:
view past answers
view problems in sets which are not open yet
see the correct answers before the answer date
see solutions before the answer date
You may want to allow TA's to act as another user but by default we restrict this to professors.
WeBWorK Main Forum
student/ta/faculty permission levels
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