образовать множественное число:
an assistant
the subject
that institute 
a figure
an angle
the cricle
a piece
the college 

an assistantsthe subjectsthats institutesa figuresan anglesthe criclesa piecesthe colleges

 assistantssubjects institutes  figuresanglescirclespiecescolleges v mnojestvennom cisle ne stavica artikli a ili an