The document discusses the proper use of "if" and "unless" in sentences. It provides examples of sentences using "if" that can be rewritten as equivalent sentences using "unless" by changing the polarity of the clause. The last sentences indicate exercises 14-17 cover more examples of rewriting sentences from "if" to "unless" by flipping the clause polarity.

1. Pages 345-346, Exercise 14
If and unless.

2. 1. You can’t enter the U.S. if you
don’t have a passport 
You can’t enter the U.S. unless you
have a passport.

3. 2. Children of immigrants will forget
their language if they don’t use it.
Children of immigrants will forget
their language unless they use it.

4. In these sentences, “if” and
“unless” are really opposite in
meaning.
If the “if” clause is negative, make
the “unless” positive.
If the “unless” clause is negative,
make the “if” positive.

5. Exercices 14-17 are all about the
same problem.