THEY - SWIM IN THAT RIVER. (-)
THEY HAVEN'T SWUM IN THAT RIVER.
HE - UNDERSTAND THE LESSON. (-)
HE HASN'T UNDERSTOOD THE LESSON.
SHE - BREAK HER LEG ONCE (+)
SHE HAS BROKEN HER LEG ONCE.
I - READ THAT BOOK (-)
I HAVEN'T READ THAT BOOK.
