Unskillful writers do not always follow the rules of parallelism. They are worried that if they repeat clauses and phrases of the same kind, their writing will be monotonous. That is not true. Of course, you are not supposed to fill your paragraphs with the same kind of sentence patterns. You must still strive to ensure that structures used within the sentence are parallel in structure.
So, for example, if you are creating a list, make sure that all items in the list have the same grammatical property. You can’t create a list consisting of nouns and adjectives. You have to choose either adjectives or nouns.
Incorrect: I like Cathie because she is beautiful, kind and intelligence. (Here we use two adjectives and one noun.)
Correct: I like Cathie because she is beautiful, kind and intelligent. (Here we use three adjectives.)
Correct: I admire Cathie for her beauty, intelligence and kindness. (Here we use three nouns.)
Incorrect: Formerly, science subjects were taught by the textbook method, while now the laboratory method is used.
Correct: Formerly, science subjects were taught by the textbook method; now they are taught by the laboratory method.
By this principle, if an article or a preposition applies to all the items in a list, it should be used only before the first item. Or you can repeat it before each item.
Incorrect: The robin sings in summer, winter, spring and in autumn.
Correct: The robin sings in summer, winter, spring and autumn.
Correct: The robin sings in summer, in winter, in spring and in autumn.