Depends on what the English sentence was. In general, "les fraises" is THE strawberries. "Des fraises" is some strawberries or just "strawberries". If someone asked you what you were eating and you said "I'm eating the strawberries", that implies the person you're talking to has previous knowledge of the strawberries (maybe they told you not to eat them or something). If you said "I'm eating strawberries" it doesn't imply any previous knowledge, just that you are infact eating strawberries.
Not in French; it doesn't have a continuous form. You can usually tell by context if it's "I eat" or "I am eating", but of course the context is missing here on Duolingo.
If you really want to stress it, you could say "Je suis en train de manger des fraises." for "I am (in the process of) eating strawberries." but it is rarely used.