The answer to the blank is "it," so the complete sentence would be like, "When you leave the bread out, it gets old."
It is used in cleft sentences. It emphasizes the main clause's subject or object. In our case, bread is the object.
The word "it" can be used as a subject (or dummy subject) in phrases describing times, dates, and weather (for example, It's raining) as well as in some idioms (It's OK).
It is often referred to as ambient "it" or empty "it." Dummy it, unlike the conventional pronoun it, refers to nothing; it only fulfills a syntactic function.
Check out the link below to learn more about subject usage;
https://brainly.com/question/846247
#SPJ1
