roads
Roads and aqueducts were abandoned in France and remained in use for years after the Romans left.
The Romans left roads because they wanted the roads so that they can get to other places of Britain easier and quicker.
Their baths, heating systems, roads, and beautiful monuments and art that is left behind.
Their organs.
The Byzantine Romans left Palestine in 634 because it was conquered by the Arabs.
nothing
There are a number of indications that would let you know that the Romans were once in Britain. This can be seen in various forms of architecture and some of the ancient literature about Romans in Britain among others.
to stay behind or to be left
The past tense of "leave behind" is "left behind."
Notes Left Behind has 272 pages.
If you mean to say 'left behind' as in, "When I moved, my friends, school, and home were left behind" Then you could substitute, 'left behind' with 'remained in the past', and remove the word 'were'.