Ambiguity of infinitive vs Gerund

Question

I don’t understand how the infinitive is less ambiguous than the gerund. Is it because the infinitive is directly associated with the verbal structure while the gerund could hypothetically be separated from the verbal structure by a comma? La escuché, (while I was) cantando en la ducha. 
Regarding incomplete and complete actions: Let’s say you’re hypothetically talking to someone about Carlos.
 He visto a Carlos fumar. 
I saw Carlos smoke. (complete action) Does this mean, I saw Carlos smoking. (But now he’s back at his desk. (action complete))
 
He visto a Carlos fumando.I saw Carlos smoking. (action in progress) While this means I saw Carlos smoking. (He is still smoking outside if you are looking for him. (action in progress)) 
 
Thanks!

Silvia Piriz · Accepted Answer

Hola Devin P.
Great question! You’ve captured the key distinction really well.
When we use the infinitive (He visto a Carlos fumar), the focus is on the complete action. It conveys that you witnessed the act of smoking at some point, but it doesn’t emphasize whether the action was still in progress when you saw it. The infinitive reports the event as a whole, as if you are summarizing what you saw.
By contrast, when we use the gerund (He visto a Carlos fumando), the focus is on the action in progress at the moment of perception. This construction highlights that you saw Carlos while he was actually smoking. The emphasis is on the action unfolding before your eyes at that specific time.
In short, the difference is:

Infinitive = complete action/event observed.

Gerund = ongoing action observed.

Your example illustrates this perfectly:
He visto a Carlos fumar → I saw Carlos smoke (action completed, considered as a whole).
He visto a Carlos fumando → I saw Carlos smoking (action in progress at that moment).
That’s why the infinitive is sometimes described as “less ambiguous”. It treats the action as a unit, while the gerund provides a snapshot of the action happening right then.
I hope this clears up the difference!
Saludos
Silvia