"I cannot love him."
Translation:Mi ne povas ami lin.
Whenever you have two or more verbs in a row, only the first is conjugated; the rest remain in the infinitive.
(English has a similar rule, by the way. Change the above sentence to "She cannot love him," and you'll see that the word "love" is in the infinitive as well; otherwise it would be "She cannot loves him," which is grammatically incorrect.)