When a/an precedes a parenthetical aside (sometimes seen in informal/conversational writing), should the vowel rule depend on the first word in parentheses, or the next word in the "regular" flow of the sentence?
I need a (memorable) idiom (preceding an m word; use a)
or
I need an (memorable) idiom (preceding an i word; use an)
The example given is not parenthetical:
(i) I need a (memorable) idiom.
A parenthesis is a remark which you insert into the middle of a sentence as if you are interrupting yourself. A parenthesis contributes to the meaning of the sentence but interrupts and stands outside its syntax. In writing, we typically use curved brackets, dashes, or commas to mark a parenthesis.
The syntax of the example sentence is not interrupted by the word memorable. Instead, the word memorable functions as an adjective modifying idiom. Consequently, the pronunciation rule applies to the word memorable and the article to use is a.
Compare this variation:
(ii) I need an (well, if I need anything at all) idiom.
Not an example of great writing, to be sure. But it shows how a parenthesis interrupts and stands outside the syntax of a sentence. The phrase “well, if I need anything at all” is not part of the noun phrase “an idiom”. The pronunciation rule still applies, but it applies to the word idiom and the article to use is an. This is true even though you would not normally pair an with well. You would, for instance, say:
(iii) I need a well known idiom.
The difference is that well is parenthetical only in example (ii) above.
The a/an rule is based purely on sound. Would you say the words inside the parentheses if you were reading the sentence out loud? If yes, then you use the first word in the parentheses to decide whether or not to use an or a. If not, then—wait, what do you mean you wouldn’t say the words in parentheses?!
A bracket only means that the word or phrase inside is less important or in some cases less relevant. But in any case that does not exclude bracketed words from the general sentence structure. Therefore, in this case the preferred way is:
I need a (memorable) idiom.
But you must understand that this is only the preferred way, not necessarily the right way. English is a rapidly growing and changing language and has numerous styles and methods. Sometimes more than one way can be called "right". And in my opinion both examples may be correct grammatically.
You choose "an" vs. "a" based on the following word, be it in brackets or parentheses or anything else. Having the word in (parentheses) doesn't change this any more than having a word in "quotes" or italics does.
As a practical matter, I try to match the a/an status of first word enclosed in parentheses and the first word on the other side of them. Thus, in this instance, I might try:
I need an (easy-to-remember) idiom.
or:
I need an (unforgettable) idiom.
If no available alternative adequately expressed my intended meaning, I would simply drop the parentheses:
I need a memorable idiom.
or rework the sentence to avoid the break after the indefinite article:
I need an idiom (memorable preferred).
I call my policy "practical" because I suspect that a mismatch—justifiable or not on appeal to logic or to outside authorities—would be distracting to a certain number of readers, and I would rather have them focus on what I'm trying to say than ponder whether I've used the right indefinite article.
Since the trick of enclosing a word or phrase in parentheses for effect isn't high on my list of essential literary tools, I don't mind using it after a/an only when the preceding indefinite article suits both the first word in parentheses and the first word beyond them.
Parentheses are meant to be optional; the sentence should make sense both with and without the "memorable". So neither option will work.
Instead, try:
I need a memorable idiom.
I need a (preferably memorable) idiom.
I need an idiom (which must be memorable).
I need a (memorable) way of putting things.
It all depends on how you would read it aloud. In your example you want "an". Reading this aloud you would not notice the bracket and instead want to hear "an easy".
If you had a parenthetical phrase rather than a tightly coupled adjective you might want to keep the article tied to its eventual partner. For instance, "He provides a (actually he doesn't provide much of anything and I hate his stupid guts) path to information." In this case, it sounds like you've broken off from the main idea of the sentence and your parenthetical expression is more of an apostrophe.
In real life, you'd say "I need a memorable idiom". So write it that way too.
Since it's followed by memorable, not idiom, in your second example, a is the correct choice.
In this particular structure, I have no doubt the article would go with the parenthetical: "an (easy) path," because we read the sentence with the parenthetical (that part with a pause and a 'lower tone' to signify parenthesis). We don't skip it.
