Mixed construction, in grammar, is when you start a sentence one way (with a certain structure) and switch mid-sentence. I think one of the most common culprits of causing of mixed construction (with my students anyway) is starting sentences with prepositional phrases then treating the phrase like a subject. Here is an example:
In my desire to get as much done as possible often makes me overlook mistakes.
The easy solution here is of course to remove the "In" or add the subject "I" to the 2nd half of the sentence:
My desire to get as much done as possible often makes me overlook mistakes.
In my desire to get as much done as possible, I often overlook mistakes.
Mixed construction is usually the result of quick writing. I am a victim of this error when I have too many thoughts coming out at once. It is easily corrected by proofreading.
Faulty parallelism is similar. My personal (non-technical) definition of this is when a sentence contains a list of some sort, and the items in the list are not in the same "grammatical family." Faulty parallelism can happen in a number of ways:
INC: I like to play volleyball and swimming.
COR: I like to play volleyball and to swim.
INC: I need to the doctor to write me a prescription or shots.
COR: I either need the doctor to write me a prescription or to give me shots.
Again, faulty parallelism in sentences is most often a result of thinking and writing very quickly and letting too many ideas come out at once. Maintaining parallel structure in your writing (not just at the sentence level, but at the paragraph level as well) gives your writing a more professional tone and keeps it looking and sounding very clean.
Mixed construction in a sentence begins with one grammatical form and ends with another, resulting in confusion. In the following example, the writer begins the sentence with a dependent clause, but then shifts immediately to the verb of the independent clause:
Although he comes from a family of pickpockets does not make him a thief.
It can be corrected by adding a subject:
Although he comes from a family of pickpockets, this criminal background does not make him a thief.
Parallelism takes place in a sentence when balanced ideas are expressed by balanced grammatical forms. Thus, an idea expressed in a single word is balanced with a single word; one in a phrase with a phrase; and one in a clause with a clause. Thus, faulty parallelism takes place in a sentence when ideas of equal weight are expressed in grammatical forms of unequal weight. In the following example, the writer has failed to balance the initial (independent) clause with clauses (dependent) of equal weight:
In our society, the young feel alienated, because they are not trusted by adults, their unfavourable representation in the media, and the capitalist economy supposes them consumeristic.
It can be corrected by righting the grammatical balance:
In our society, the young feel alienated, because they are not trusted by adults, they are represented unfavourably in the media, and they are considered consumeristic by the capitalist economy.