Be动词后可以有动词。
Be动词后可以有动词，但只能是正在进行时或者构成被动，也就是用doing或者done。
do you understand错的原因是
be动词后面不可能用动词原形，一个句子只能有一个动词作谓语。
forget to do something = 忘记去做某事. 例句: I forget to lock the door. 我忘记锁门了.
forget doing something = 忘记做了某事. 例句: I forget locking the door. 我忘记(已经)锁过门了.
remember to do something = 记得去做某事. 例句: Remember to lock the door. 记住要锁门.
remember doing something = 记得做过某事. 例句: Remember locking the door. 记住(已经)锁过门了