I can't understand the note duration notation very well.

In the image below, the highlighted A in the second measure seems to start half a beat between the first and the second beat. But shouldn't we wait for the bass A two beat duration to finish, and only then hit the second A? How do we know when the second A should start? In this case I know intuitively (knowing how the piece sounds) that it should start half a beat after the bass A. How would we write these notes if we wanted the second A to start only after the bass A finishes its duration?

[![Enter image description here](https://i.stack.imgur.com/ebvZ0.png)](https://i.stack.imgur.com/ebvZ0.png)