"Mixed construction" is a fancy way to say that your sentence doesn’t make sense. A mixed construction happens when you start a sentence with one construction and finish it with another.
At their most basic level, sentences have a subject and a predicate. In a mixed construction sentence, the predicate doesn’t match the subject.
Let’s take a look at an example:
The construction of this sentence doesn't make sense. It starts off looking like it will follow one grammatical direction, but ends up in an entirely different (and incorrect) place.
Here's how you could fix it: