Ger- is the Latin root of 'gerund'. A Latin derivative of the Latin root syllable is the infinitive 'gerere', which means 'to carry about'. An English derivative, by way of the preceding Latin derivative, is the noun 'gerund'.
The root that means 'severe' is from the ancient, classical Greek and Latin languages. That root is auster- in Latin, and austeros in Greek. From that root derive the Latin adjective 'austerus', which means 'severe'; and the Latin noun 'austeritas', which means 'severeness, severity'.
Scire is the Latin equivalent of 'to know'. The Latin infinitive derives from the Latin root sci-, which means 'to know'. Examples of English derivatives of the Latin root are the noun 'science' and the adjective 'scientist'.