All sentences are comprised of at least two parts, a subject and a predicate. The subject is the noun which the sentence is about. For example, in the sentence:
“Over reading break, the student worked on his paper”
The student would be considered the subject. The predicate, meanwhile, is the action of the sentence. It contains a verb and describes what the subject is doing/what is being done to the subject. So, in the above example, worked on his paper would be considered the predicate of this sentence.
By contrast, a mixed sentence is missing a part. It either has no subject or has two subjects and no predicate. For example:
“Over reading break, worked on his paper”
This sentence is missing a subject, leaving the sentence incomplete. In another example:
“Over reading break, the student and his paper.”
This is missing a verb, and thus the predicate. It is equally incomplete.
This is a little bit different than a sentence fragment, which is only incomplete. Mixed sentences often result when the writer starts the sentence with one plan and ends it with another plan, resulting in some missing parts along the way.
For example, in the sentence:
“Leah, an interesting companion, someone who would stand up for me.”
The predicate is missing. It is uncertain what Leah is doing in the sentence. Rather, she is simply being passively described.
“When I went for a run also went to the store near my house to get eggs.”
Here, the subject is missing. While not every sentence you write or read will be that simple, what’s important to keep in mind is: what is the action in this sentence, and who or what is doing the action/having the action done to them? As long as you have both of those, you have avoided mixed construction.