Breakdown of Þú mátt ekki sleppa kaðlinum.
þú
you
ekki
not
mega
to be allowed to
sleppa
to let go, to escape / to avoid / get out of
Questions & Answers about Þú mátt ekki sleppa kaðlinum.
Why is the rope (kaðlinum) in the dative case here?
The verb sleppa governs the dative case when it means to let go of or to release an object.
The dictionary form of rope is kaðall. Why does it change so much to become kaðlinum?
The masculine noun kaðall drops its second 'a' when case endings are added. This makes the dative singular form kaðli. When you attach the dative masculine definite article -num, you get kaðlinum (the rope).
I learned that mega means may. Why is mátt ekki translated as must not?
While mega usually means may or to be allowed to, the negative phrase mega ekki expresses a strict prohibition, mapping perfectly to the English must not. If you wanted to say you do not have to do something, you would use þurfa ekki instead.