"개미는 오늘도 열심히 일합니다."
Translation:The Ant works hard today too.
It's the particle 도 in 오늘도 so today also/too/as well. The problem with 'also' especially is it could apply to other parts of the sentence besides today -- perhaps the ant, or that it works hard -- depending on where the emphasis is placed in the spoken English. "The ant works hard today also." "The ant works hard today also." Try that with 'too' or the more formal version 'as well' and the meaning doesn't change so much.
"The ant also works hard today." Here also more likely modifies the ant or that it works hard, as it's quite distant from today. In this case, to be clear that it's today as opposed to other days, though there are surely other ways to say 'again' in Korean, in English I'd go with, "The ant works hard again today." Even that could mean it works hard more than once in the day, though . . .