Sous le microscope, la paroi est tapissée de fibres.

Questions & Answers about Sous le microscope, la paroi est tapissée de fibres.

Why do we use de instead of par for the passive agent here?
In the passive voice, the agent is usually introduced by par. However, we use de when the verb describes a state or condition rather than a specific physical action performed by an active participant. Here, est tapissée de describes the physical state of the wall being covered, so it takes de.
Why use la paroi for 'the wall' instead of le mur?
Le mur refers to a standard architectural wall, like the brick or drywall in a house. La paroi refers to an inner surface, partition, or lining. It is heavily used in science, biology, and anatomy to describe the walls of cells, organs, or cavities. Since this sentence involves a microscope, la paroi is exactly the right word.
Why is it de fibres and not des fibres?
The passive phrase here requires the preposition de (to be lined with). When the preposition de is followed by a plural indefinite noun (which would normally use the plural article des), the des drops out and we just use de. This indicates the wall is covered with fibers in a general sense.
Why does tapissée end with an extra -e?
This sentence is in the passive voice, which is always formed using the auxiliary verb être plus a past participle. In this structure, the past participle must agree in gender and number with the subject. Because la paroi is feminine singular, tapissé becomes tapissée.

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