After verbs of sense perception (to see, to hear, to feel, to watch, to observe, to notice and some others). In this case the only possible form of the infinitive is the non-perfect common aspect active voice form, used without the particle to:
No one has ever heard her cry. I paused a moment and watched the tram-car stop.
The verb to listen to, though not a verb of sense perception, is used in the same way, with a bare infinitive: He was listening attentively to the chairman speak. [Кобрина].