"I'm really interested in this class."
Translation:A mí me interesa mucho esta clase.
Me interesa is enough context that you can leave off the 'A mi'.
The 'A mi' just gives further context to who you're referring to.
For example "le interesa mucho esta clase" could be He or She are really interested in this class. Unless the he or she were apparent from the context of the conversation, you would need the A él or a ella where the a mi is now to specify.
I hope I'm not too confusing.
The 'me interesa' is the important part her. It is whats saying "I have interests/I am interested/etc".
The a mi is just redundancy. It specifies who you're talking about. This is just Duolingo trying to get you used to the idea.
These are all pretty self explanatory and don't really require the redundancy. I have interest, you have interests, we have interest.
But le interesa would need it (unless the subject is clear from the conversation).
It could mean he, she, you (formal and it I think).
Likewise, les interesa could mean they (ellos or ellas) or the plural you (ustedes).
I had "Me interesa mucho en esta clase." But Duo says it's wrong. I don't think I have to have "A mí" at the beginning all the time. Is it why because this sentence has "really", so that "A mí" will emphasize the sentence? I reported to accept my answer, tho...
This verb works like gustar. It doesn't point to who is doing it but who or what it is being done to.
The conjugation has to match what is interesting.