In nonidimatic writing (nonidiom), the message of a phrase or sentence is clear, but a native speaker would not phrase the idea that way.
- In the sentence: When I write under pressure, I feel that I lack of knowledge of expressing myself with sophisticated English words,…
The underlined part is not written in idiomatic English.
A native speaker might pharse the sentence in this way: When I write under pressure I fell that I cannot use sophisticated English words to express myself or I lack the knowledge to express myself in sophisticated English.
"Nonidiomatic" differs from "unclear" in that the reader can usually grasp the meaning in a nonidiomatic phrase or sentence but not in an unclear phrase or sentence. Nonidionmatic errors are usually local (less serious); however, if too much of a piece of writing is nonidiomatic, the error can become global (more serious).
Errors with idiomatic expressions differ from nonidiomatic writing. Indiomatic expressions are set words or phrases that are commonly used and that can be found in a dictionary of idioms or a text that focuses on idioms. Various errors can occus with idiomatic expresstions.
- In this sentence: I would rather use a computer than write by a hand,
by a hand should be by hand, an article error.
- In the sentence: I make the same mistakes in English times after times, times after times
Should be time after time, asingular-plural error.
 nonidiom = grading symbol for nonidiomatic writing