Help on reading/interpreting/writing notes. How do I do this?

Hi.
I'm rather new to MuseScore and lately, I've been trying to learn the program - and I feel I've made huge progress, but now I'm stuck... The attached image shows a measure somewhere near the end of the 3rd Movement of Beethoven's Moonlight Sonata. I have no idea of how to interpret the notes shown - and no idea how to put them into my "training-score" in MuseScore either. I'm familiar with voices and how to put the notes in, but I have no idea what the notes written on the attached image mean... how are they played/written? When I try to summarize the note-values, they exceed the four available "beats" (I'm terribly sorry about my lack of correct "lingo" here, but I'm learning...). If someone with a bit more experience in both note-reading and score-making than myself could explain this to me, like I was 6 years old, I would be very grateful.

Best regards