In the sentence “Helen is taking an online course on digital drawing,” the indefinite article “an” introduces the noun phrase “online course” as a non‑specific, general example of that kind of thing, not a unique or previously known one.

More specifically, in that sentence “an” is used to:

  • Show that “online course” is singular and countable (you can have one course, two courses, etc.).
  • Indicate that it is not a particular, previously mentioned course; it could be any online course on digital drawing.
  • Fit the sound rule for indefinite articles: we use “an” before a word starting with a vowel sound (“online”), rather than “a.”

So the function of “a/an” there is: to mark “online course” as a single, countable, but non‑specific item of that type.