When you write a series of elements in a sentence, each element must (1) appear in the same grammatical form and (2) perform the same grammatical function. This is the rule of parallel construction. If any element fails to satisfy either criterion, you have written a nonparallel construction.
You'll join elements in a series with these coordinating conjunctions: but, or, yet, and. You may also use the correlative conjunctions, which come in pairs: either … or, neither … nor, not … but, both … and, not only … but (also). With the correlative conjunctions, you must make certain that the element joined by the first word (e.g., not only) grammatically mirrors the element joined by the second word (e.g., but or but also).
Note: The art of parallel structure is thoroughly discussed in the eBook Developing a Powerful Style.
When you join three elements or more with a conjunction, you should put a comma before the conjunction. This is called the serial-comma rule. Thus: red, white, and blue.
Here is a nonparallel construction formed with a coordinating conjunction:
He is determined, forceful, and a natural leader. (the word leader is a noun; the other elements in the series are adjectives) You could fix the above mistake by tripling the adjectives:
He is a determined, forceful, and natural leader. Here is a nonparallel conjunction formed with a correlative conjunction:
The manager ordered not only a full report but also wanted some background research. (The not only joins a noun acting as the direct object of the verb ordered, but the but also joins another verb wanted.) You could fix this mistake by joining the two verbs with not only … but also.
The manager not only ordered a full report but also wanted some background research.