Generalizations with c'est (c'est dur)

Generalizations with c'est

Normally, adjectives in French agree in gender and number with the noun they describe. However, to make a broad generalization about a material, category, or concept, use c'est followed by a masculine singular adjective.

The adjective stays masculine singular even if the noun you are generalizing is feminine or plural.

Granite is very compact.
Le granit, c'est très compact.
In general, rock is rough.
En général, la roche, c'est rugueux.

Notice that la roche is feminine, but the adjective rugueux stays masculine.

Limestone is porous and light.
Le calcaire, c'est poreux et léger.

Specific Descriptions vs. Generalizations

To describe a specific object rather than making a generalization, use être and make the adjective agree with the noun as usual.

The erosion is fast because the rock is porous.
L'érosion est rapide parce que la roche est poreuse.
This crystal is opaque and compact, I know it.
Ce cristal est opaque et compact, je le sais.

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