Rashi on Exodus
תשחת הארץ means the ground was being destroyed (the imperfect tense denotes what was then going on continuously), and so the Targum has אתחבלת ארעא (the participle) the ground was being ruined.
Or HaChaim on Exodus
תשחת הארץ, the land was ruined. This means that the beasts would destroy everything except the human beings.
Siftei Chakhamim
אתחבלת ארעא (past tense). Rashi is answering the question: Why is [it written] תשחת הארץ a term that denotes the future tense, if the land had already been devastated? Therefore Rashi cites Onkelos, who explains the term as אתחבלת ארעא , to prove that this term also denotes the past or present tense.