Is there a difference between except if and unless? Which is more common?
- Unless it rains tomorrow we are planning on going to the park.
- Except if it rains tomorrow we are planning on going to the park.
In my experience, when we exclude technical books on software and programming, and focus instead on other kinds of prose, it is unusual for a sentence to start with Except if. Most often, an except if-clause is added on as a qualifier after the matrix clause rather than used as an opening condition. On the other hand, unless-clauses appear before and after the matrix clause.
They're pretty much the same thing. Unless is more common and combines condition with exception (except + if), hence the negative meaning; provided (that) and its synonyms are the positive equivalent. Except if sounds clunky on its own but works well enough with preceding context:
“But it always says something like that on the boxes you get games in,” said Johnny. Except if you get them from Wobbler, he added to himself, when you just get a disk.