decem

Decem is indeclinable, like every Latin cardinal from quattuor to centum. Only unus, duo, tres and the hundreds above ducenti take endings. So the number stays put and the noun does the grammar: decem dierum, decem militibus.

Its family is large and mostly obvious once seen: decimus, "tenth", decuria, a group of ten, denarius, the coin originally worth ten asses, and December, which was the tenth month before January and February were bolted on the front. The trap worth knowing is decimare: it means to punish one man in ten, not to wipe out most of a group, and the modern English "decimate" has drifted a long way from the arithmetic.