Complex sentence: However, with 100 tests where all null hypotheses are true, the expected number of incorrect rejections is exactly 5, and if the tests are independent the probability of at least one incorrect rejection is 99.4%.
Split into simpler sentences: However, with 100 tests where all null hypotheses are true, it is highly likely that at least one null hypothesis will be rejected incorrectly. In fact, the expected number of such rejections is exactly 5.0 if the tests are independent, and the probability of at least one rejection is 99.4%.