Certain verbs are followed by the gerund, certain verbs followed by the infinitive, and some verbs can be followed by either (sometimes with a difference in meaning, sometimes not). In general, the gerund is used when the second verb talks about something that occurs at the same time or in the past. The infinitive is used when the second verb is something that is only possible or will happen in the future. There are of course many exceptions but this is a general rule. Here is a good list of common verbs and which form they are followed by: http://www.perfect-english-grammar.com/support-files/gerund_infinitive_verbs_list.pdf Is suspected of having stolen the money - because the stealing happened in the past. He is endeavoring to help the Third World - he will help in the future/it is only a possibility at this point The players must train their ability in heading the ball wherever they want to put it. The players must train their ability to head the ball wherever they want to put it. Both can be correct They didn’t stop until having spent every last penny of it. They don’t stop until they have spent every last penny of it. The first one could be correct if "didn't" was in the past tense. This is a little confusing because of the use of simple present, which means it is something that happens regularly.