请大家帮帮忙啊，几道语法！！

shinnaUID: 2105519

Franklin D.Rooserelt was ___the great force of radio and the opportunity it provided for taking govenment policies directly to the people.
答案选的是the first president to understand fully
不明白为什么这里要加个to,为什么不能用the first president understanding fully

Eleanor Roosevelt set the standard against which the wives of all United States Presidents since have evaluated.
答案错在最后的 have evaluated ,但还是不明白应该怎么改？

Physical fitness activities  can lead to an alarming variety of injuries if participants push themselves greatly hard.
答案错在greatly,但不知道错在哪里。
查过字典了，greatly 也有表示“非常的”的意思！！！为什么不能用在这里！！

请大家帮忙

vincentfjUID: 134258

第二题把have去掉