the only things he could do was call his wife=HE COULD ONLY CALL HIS WIFE·或者·he could do nothing but call his wife
但如果是HE HAD NO CHOICE BUT 的话,后面就要加TO DO,如：he had no choice but / except to call his wife
还有what he wants to do is to see you
WHAT HE CAN DO IS STAY AT HOME AND NOT GO OUT
能与CAN连接的是不带TO的动词原形,所以这里的STAY前面就不带TO.
he enjoys visiting his friends=what he enjoys doing is visiting his friends,
is/ was 后面的动词形式由is/ was前面的具体动词来决定.再看以下例句
the only thing he did was (to) call his wife
the only thing he could do was call his wife
the only thing he wanted to do was to call his wife
the only thing he enjoyed doing was calling his wife
在his hobby is to collect / collecting coins 这个句子中 is的前面没有动词,所以to collect和collecting都可以用