How do I know if it's plural? And why do they speak so fast? Is that really how they sound in their language?
If it was one egg, it would be "Ett ägg". And yes, native speakers tend to speak fast.
Yes, the ett-words that are the same in singular and plural indefinite will most often have the indefinite article ett if singular.
Is there an explanation to why they give "The boy has an eggs." as a correct solution?