"The big dog runs into the sea."
Translation:A nagy kutya befut a tengerbe.
I believe the prefix on the verb marks it as a more "finished" action.
In German, too, one could say both Der große Hund rennt ins Meer. or Der große Hund rennt ins Meer hinein. - once with prefixless verb rennen and once with prefixed verb hineinrennen - even though ins already indicates direction.
It sort of emphasises that the dog actually ended up in the water, while the first one might also be used while he is still running and is perhaps still on the beach and not in the water yet at all.