"She beat the big drum."
Translation:Hon slog på den stora trumman.
"She beat the big drum" is a past sentence since it's "beat", not "beats" (the rule of the "s/es" for the present 3rd person singular), it's not ambiguous.
I'm not native English, so I have really big problems with verbs like "let" or "beat" too, even if I know the rule. I don't often see that small detail when I'm using Duo =/
And if it were present (without any context), yes, "hon slår på..." = "she's beating...". I still struggle with the regular present, "håller på" and "sitter/står/... och...", but all of them can be different kinds of present continuous in English (If I am not wrong).