Called the "bare infinitive". Use your favorite search engine, many sites will explain it better than I can. Short: "Help is a verb that can be used with or wi

Called the "bare infinitive". Use your favorite search engine, many sites will explain it better than I can. Short: "Help is a verb that can be used with or without to and with or without an object before the infinitive. When we use it without an infinitive it sometimes sounds more informal." (from Bare infinitive - Learning English | BBC World Service)