what is the difference between "eisc" and "iasc"?
éisc is plural and genitive singular.
I know of two exceptions: grásta is both a nominative singular form and a nominative plural form (as well as a genitive singular form), and bó is both a genitive singular form and a genitive plural form (as well as a nominative singular form).
Just edited that statement out.
Not to be nitpicking 'cause I know it's just for practice, but fish are animals
Why is there no an in the phrase under the surface?
The preposition is faoi. Because it ends in a vowel and an starts with a vowel, they fuse to create faoin - "under the".
If the noun was plural, you would say faoi na - faoi na tithe - "under the houses".