asper

Asper, aspera, asperum declines like bonus, but it is one of the -er adjectives that keeps the e right through: aspera, asperum, asperi. Compare pulcher, pulchra, which drops it. There is no rule to spare you here; the two groups have to be learned as lists.

The range runs from physically rough surfaces out to weather, temper, prose style and fortune, so res asperae is "hard times" and nothing to do with texture. Grammarians used it for the rough breathing, spiritus asper, which is why Greek words entering Latin with an h are said to be aspirated. English inherited asperity and exasperate, both of which have kept the abrasiveness intact.