Perceiving actions in progress
When you talk about seeing or hearing someone in the middle of doing something, use a perception verb like ver (to see) or oír (to hear) followed by the person and a gerund (an -ando or -iendo word).
This emphasizes that the action was ongoing while you observed it. Remember to use the personal a before the object if you are perceiving a specific person or people.
| I heard someone playing the melody on the piano. | Oí a alguien tocando la melodía en el piano. |
| During the tour, I heard the bassist humming that song. | Durante la gira, oí a la bajista tarareando esa canción. |
| I saw the bassist jumping on the stage. | Vi a la bajista saltando en el escenario. |
Perceiving completed actions
If you witness an entire action from start to finish, or if you simply want to refer to the action as a whole rather than an ongoing process, use an infinitive instead of a gerund.
| We heard the crowd sing the chorus. | Oímos al público cantar el estribillo. |
| It is horrible to hear someone sing out of tune live. | Es horrible oír a alguien desafinar en directo. |
When using the infinitive, you have some flexibility with word order. The infinitive can come after the person doing the action (as in the examples above), or it can attach directly to the perception verb.
| Apparently, nobody saw the amplifier fall. | Al parecer, nadie vio caer el amplificador. |
| The acoustics are so bad that we saw the people leave. | La acústica es tan mala que vimos salir a la gente. |
Notice how in the last example, the infinitive salir comes right after vimos, followed by the personal a and the people performing the action. Both word orders are perfectly natural in Spanish when talking about completed actions!