The English word ?Strength " can be translated into Latin in many different ways, according to its different meaning.
So here are these translations:
1-"VIRTUS ", if " strength " means "bravery" ,"gallantry", ?valour?.
2-"FORTITUDO" or ?AUDACIA? , ?ANIMUS?, if " strength " means "courage" as opposed to 'fear'.
3-"IMPERIUM "or ?VIRES? , if " strength " means "military strength ".
4-"VIS "or ?ROBUR?, if " strength " means "brute force", ?violence?.
5-POTENTIA? or ?POTESTAS?, if ?strength? means ?power?.
6-?FIRMITAS?, if ?strength? means ?firmness?.
All these Latin words are in the nominative case, of course.
Latin in fact is an inflectional language with five declensions and six cases and therefore VIRTUS; FORTITUDO, IMPERIUM, VIS, etc. can have different endings, according to the context of the phrase.