Perception verbs describe what we sense — seeing, hearing, feeling, noticing, observing. When one of these verbs takes an object followed by another action, English offers two patterns, and the choice between them changes the meaning.
Compare these two sentences:
In the first, you witnessed the whole thing: he walked out, door to pavement, start to finish. In the second, you caught a slice of the action while it was happening — you saw him mid-departure, not the complete event.
The verbs
The main perception verbs that take this pattern:
see, hear, watch, feel, notice, observe, listen to, look at, smell, taste.
All of them follow the same structure: verb + object + bare infinitive or verb + object + -ing form. The bare infinitive has no to.
Pattern 1 — complete action
Use object + bare infinitive when you perceive the action from beginning to end.
The emphasis is on the whole event. You saw the painting finished. You heard the door complete its slam. You noticed the car's full departure.
Pattern 2 — action in progress
Use object + -ing form when you perceive only part of the action, while it was happening.
Here you're not reporting the whole event — you're reporting a moment caught in motion. The painting may not be finished. The arguing may have started before you noticed and continued after.
The -ing form also suggests repetition or duration:
Compare
Placing the two patterns side by side makes the contrast sharp.
Both forms are grammatical. The choice is about what you want to say about the moment.
After *can* and *could*
When see, hear, feel follow can or could, use the -ing form, not the bare infinitive.
Saying I can hear someone to talk is wrong. The bare infinitive does not follow can with these verbs.
Passive forms
In the passive, the to that disappeared in the active pattern returns.
This is mostly formal or literary. In everyday English, the -ing form is far more common in passive sentences:
If you're writing a newspaper report or a formal account, was seen to leave fits. In conversation, was seen leaving is the natural choice.
A note on linking verbs
Look, sound, feel, taste, smell also work as linking verbs — followed by an adjective to describe a quality, not an action.
This is a different pattern entirely. No object, no second verb — just the adjective describing the subject. Linking verbs are covered separately.
Common mistakes
Adding to with the bare infinitive
Using -ing for a complete, finished action
Using the bare infinitive after can / could
Using a finite verb after the object
Confusing the perception pattern with the linking-verb pattern