Here's why:
* Active Voice: The subject of the sentence performs the action. Example: "The dog chased the ball."
* Passive Voice: The subject of the sentence receives the action. Example: "The ball was chased by the dog."
"Couldn't see" is a verb phrase that expresses inability, but it doesn't have a clear subject performing or receiving an action.
To determine if a sentence is active or passive, you need a complete sentence with a subject and verb.
For example, here's how you could make "couldn't see" into active and passive sentences:
* Active: "I couldn't see the road." (Subject: I, Action: couldn't see)
* Passive: "The road couldn't be seen." (Subject: road, Action: couldn't be seen)