"There are many lakes in Sweden."
Translation:Det finns många sjöar i Sverige.
I'm not a native speaker, but I'll try to clarify.
In English, 'is'/'are' can be used either to describe what something is, or simply state that it exists.
It is a lake. (What is 'it'? A lake.) There is a lake. (There exists a lake.)
Är can only mean 'is'/'are' in the first sense. Finns can only mean 'is'/'are' in the second sense.
That's not entirely correct; "är" can be used in the second sense provided a location is provided, at least that seems to be the case from other examples and how I remember them being explained. Perhaps there are further requirements that are not meet in this example, but to the best of my knowledge, är should be accepted here.