Perception Verbs + Object + Bare Infinitive or -ing

Perception verbs — verbs describing what we see, hear, feel or smell, such as see, hear, watch, notice, feel, smell, plus the phrasal verbs listen to and look at — can all be followed by an object + bare infinitive (no "to") or an object + -ing form. Both patterns are grammatically correct, but they describe the action differently.

• I saw her cross the street. (object + bare infinitive)
• I saw her crossing the street. (object + -ing)

Both sentences are correct English, and both describe the same basic event — but they are not interchangeable. The verb form you choose tells the listener exactly how much of the action you actually witnessed.

The Core Rule: Complete Action vs Action in Progress

Object + bare infinitive describes the whole action from beginning to end — a complete event that you witnessed in full.

• I saw her cross the street. (I watched the entire crossing, from one side to the other.)
• We heard him play the whole concerto. (We heard it from the first note to the last.)

Object + -ing describes an action that was already in progress, that may not have been witnessed from start to finish, or that is ongoing/continuous.

• I saw her crossing the street. (I saw her in the middle of crossing — I may not have seen her start or finish.)
• We heard him playing the piano when we walked in. (The playing was already happening; we caught it in progress.)

The -ing form can also suggest that the action was repeated, habitual, or simply longer in duration, whereas the bare infinitive usually implies a single, complete event witnessed as a whole.

More Examples: Bare Infinitive vs -ing

Object + bare infinitive (complete action)	Object + -ing (action in progress / partial)
I watched the plane take off.	I watched the plane taking off as we drove past the airport.
She noticed him leave the room.	She noticed him leaving the room and wondered why.
I felt the ground shake for a few seconds, then stop.	I felt the ground shaking beneath my feet.
We heard the door slam.	We heard someone shouting outside for ages.
I smelled the bread burn.	I could smell something burning in the kitchen.
They watched the sun set behind the mountains.	They watched the sun setting as they talked.
I saw him drop his phone.	I saw him walking down the street, talking on his phone.

The Passive Voice: A Key Exception

When a sentence with object + bare infinitive is turned into the passive voice, the bare infinitive changes to a to-infinitive. This is one of the most commonly tested rules with perception verbs.

• Active: I saw her cross the street. → Passive: She was seen to cross the street.
• Active: We heard him play the concerto. → Passive: He was heard to play the concerto.

The -ing form, however, stays exactly the same in the passive — no "to" is added:

• Active: I saw her crossing the street. → Passive: She was seen crossing the street.

Rule Summary Table

Pattern	Voice	Meaning	Example
Object + bare infinitive	Active	Complete action, witnessed start to finish	I saw her cross the street.
Object + to-infinitive	Passive	Same complete action, passive voice	She was seen to cross the street.
Object + -ing	Active	Action in progress / partial / ongoing	I saw her crossing the street.
Object + -ing	Passive	Same action in progress, passive voice (no change)	She was seen crossing the street.

Direct Perception vs Reported Perception (that-clauses)

Perception verbs followed by object + bare infinitive/-ing describe direct sensory perception — something you physically saw, heard, felt or smelled happening. This is different from using a that-clause after a perception verb, which usually reports an inference, opinion or mental observation rather than something perceived directly with the senses.

• I saw her cross the street. (direct sensory perception — I watched it with my eyes)
• I noticed that she seemed tired. (a mental observation/inference, not something you directly watched happen — "seeming tired" isn't a single physical event you can witness the way you witness someone crossing a street)

Both structures are correct, but they are not simply two ways of saying the same thing: the object + infinitive/-ing pattern reports what your senses directly registered, while the that-clause reports a conclusion or perceived state.

Common Mistakes

• Adding "to" in the active voice: The bare infinitive never takes "to" after a perception verb in the active voice.
  Wrong: I saw her to cross the street.
  Correct: I saw her cross the street.
• Forgetting to add "to" in the passive voice: When the bare-infinitive pattern is made passive, "to" must be inserted.
  Wrong: She was seen cross the street.
  Correct: She was seen to cross the street.
• Adding "to" to the -ing form in the passive: The -ing pattern never changes in the passive.
  Wrong: She was seen to crossing the street.
  Correct: She was seen crossing the street.
• Ignoring the meaning difference: Choosing the bare infinitive when you only witnessed part of the action (or vice versa) misrepresents what you actually perceived. If you only caught the middle of an action, the -ing form is the accurate choice.

Practice Tips

• Practise transforming to the passive: Take active sentences with object + bare infinitive and rewrite them in the passive, remembering to insert "to." Then do the same with -ing sentences and notice that nothing changes.
• Collect real examples: Note down sentences with see/hear/watch/notice + object from books, films or conversations and decide whether the bare infinitive or -ing was used, and why.
• Compare with "that": Practise rewriting object + bare infinitive/-ing sentences as that-clauses (where possible) and discuss how the meaning shifts from direct perception to a reported observation or inference.

Practice Exercises

Explore related grammar topics:

All Grammar Topics Causative Verbs Verb Patterns Gerunds vs Infinitives Participle Clauses Passive Voice

Frequently Asked Questions

What are perception verbs in English grammar?

Perception verbs describe what we perceive with our senses: see, hear, watch, notice, feel and smell, along with the phrasal verbs listen to and look at. They can be followed by an object plus a bare infinitive (e.g. "I saw her cross the street") or an object plus an -ing form (e.g. "I saw her crossing the street"), and the choice between the two changes the meaning.

What is the difference between "I saw her cross the street" and "I saw her crossing the street"?

"I saw her cross the street" means you saw the complete action from start to finish — the whole crossing. "I saw her crossing the street" means you saw her in the middle of the action, in progress, and you may not have seen the beginning or the end. The bare infinitive reports a complete event; the -ing form reports an action already underway or only partly witnessed.

How do you make the passive of "I saw her cross the street"?

In the passive voice, the bare infinitive after a perception verb changes to a to-infinitive: "She was seen to cross the street." This is an important exception to the usual "no to" rule for perception verbs, and it only applies to the bare-infinitive pattern, not the -ing pattern.

Does the -ing form change when you make a perception-verb sentence passive?

No. The -ing form stays exactly the same in the passive voice. "I saw her crossing the street" becomes "She was seen crossing the street" — no "to" is added, unlike with the bare infinitive, which does require "to" in the passive.

Is "I saw her to cross the street" correct?

No, this is incorrect. In the active voice, perception verbs like see, hear, watch and notice are followed by the bare infinitive with no "to": "I saw her cross the street." Adding "to" in the active voice is one of the most common mistakes learners make with this structure. "To" only appears once the sentence is made passive.

Which verbs follow this object + bare infinitive/-ing pattern?

The main perception verbs that follow this pattern are see, hear, watch, notice, feel and smell, as well as the phrasal verbs listen to and look at. All of them can take either an object + bare infinitive (for a complete action) or an object + -ing form (for an action in progress).

What is the difference between "I noticed her leave" and "I noticed that she seemed tired"?

"I noticed her leave" describes direct sensory perception — you physically watched her leave the room. "I noticed that she seemed tired" reports a mental observation or inference about her state, not a single physical action you watched happen from start to finish. The object + infinitive/-ing pattern is used for direct perception, while a that-clause typically reports an inferred observation.

Can the -ing form describe a repeated or habitual action?

Yes. Besides describing an action caught in progress, the -ing form after a perception verb can also suggest that the action was repeated, continuous or longer in duration, such as "I keep hearing him complaining about the traffic." The bare infinitive, by contrast, usually describes a single, complete, often shorter event witnessed in full.

Is this the same as causative verbs like make, have, let and get?

No. Causative verbs (make, have, let, get someone do something) describe causing or allowing someone else to perform an action. Perception verbs (see, hear, watch, notice, feel, smell) describe what you directly witnessed happening, using the same object + bare infinitive/-ing structure but for a completely different meaning. See our separate causative verbs guide for that pattern.

At what level is this perception-verb structure typically taught?

This structure, especially the meaning contrast between the bare infinitive and -ing and the passive to-infinitive exception, is typically introduced at B2 level and refined at C1, since it requires learners to already be comfortable with basic verb patterns, infinitives, gerunds and the passive voice before tackling the nuance between the two forms.