"Zij komen hierna."
Translation:They come after this.
Are they coming after something as in a time. So: they come on the next bus.
Or are they coming after an object. So: they are coming after this ring (my precious...)
Or is it either?
An educated guess - because these would ask for what (after these messages, e.g.) and this can stand on its own (after this). At least that's how it is in my native tongue (Serbian). As in Dutch, when many things come together in a group we refer to all of them together in singular (English all of this is like that).
"...after this." isn't the best way to understand it. I think the best translation is "hereafter", which makes it a lot easier to comprehend since they are both one word.