为什么下面3个if句(选自of studies)动词都用原形，实在没人可问，请多多指教！

为什么下面3个if句(选自of studies)动词都用原形，实在没人可问，请多多指教！
1、If his wit “be” not apt to distinguish or find differences, let him study the Schoolmen;
2、if a man “write‘ little, he had need have a great memory;
3、if he ”confer“ little, he had need have a present wit;