Pr.cont: Tricks are being performed by clowns at the circus.
Past simp: Tricks were performed by clowns at the circus.
Past cont: Tricks were being performed by clowns at the circus.
Fut.simp: Tricks will be performed by clowns at the circus.
Pr.perf: Tricks have been performed by clowns at the circus.
Past perf: Tricks had been performed by clowns at the circus.
Fut. Perf: Tricks will have been performed by clowns at the circus.
Modals: Tricks can be performed by clowns at the circus.