When is a comma necessary to set apart a subordinate clause

If you have the subordinate clause before the main clause, you write a comma. However, the rules are a bit different for relative pronouns connecting main and subordinate clause.

ich statement best shows a problem in travels with charley?

one sharp difficulty presented itself. in the intervening twenty-five years my name had become reasonably well known.

this being so, my trip demanded that i leave my name and my identity at home.

i had to go alone and i had to be self-contained, a kind of casual turtle carrying his house on his back.

with all this in mind i wrote to the head office of a great corporation which manufactures trucks.