1. here and there, 2. here and now, 5. there and then, 6. now and then
"Here and then," and "there and now" are awkward and don't make much sense. But the other four are what we call idioms. They have commonly accepted meanings that are a little more precise than the literal definitions:
"Here and there" means scattered about in various places: "Strange rock formations can be found here and there."
"Here and now" means pretty much what it says, in this place and at this time: "We are going to settle this matter here and now."
"There and then" generally refers to a particular place at a time in the past: "Right there and then they agreed to the terms and conditions."
"Now and then" might be defined as occasionally: "Now and then I like to have a bowl of ice cream."
The title of this page is "There and Then." This is in contrast to the previous page, "Here and Now." Since "here and now" referred to the center of the universe, "there and then" could mean the perimeter, or at least some point away from the center. However, the cosmologists tell us that the universe has no edge. The analogy they use is the concept of a two dimensional universe on the surface of a sphere. No matter which direction or how far you might travel across this imaginary cosmos, you will never find an edge. So that leaves us with somewhere else as a working definition for "there and then." For our everyday life here on Earth, that's fine. "There" is any place that is not "here." "Then" is any time, past or future, that is not "now."
Remember, here and now are relative terms. Here can include more than just your house, or your town. It can mean the whole Earth, or the solar system, the galaxy or the universe. If here is the entire universe, then there is no there. Similarly, now doesn't have to mean this minute or today. It can be this year, this century, this millennium or forever. If now is forever, there is no then, is there?