Are there any specific rules for these particles—
These kinds of particles have two variants,
- one for those with a final consonant (e.g. “잭” — “은,” “이,” “을,” “과,” “-이다,” “-이면,” “-이라서,” “-이라면,” “-이고,” “-이었-,” “-이에요,” and “-아.”),
- and another for those without (e.g. “사과” — “는,” “가,” “를,” “와,” “-다,” “-면,” “-라서,” “-라면,” “-고,” “-였-,” “-예요,” and “-야.”).
It depends on how the preceding word sounds when you read it.
—when the preceding word is not written in Korean script
You completely transliterate it into Korean first and pick the right particle variant accordingly. “Cat,” for example, becomes “캣” (or “캐트” by some speakers). It’s not “캩” because that’s just how the /t/ at the end usually gets transliterated by native South Koreans nowadays(I don’t know how the North’s doing on that.). For that, South Korea has the standard way of transliteration, “외래어 표기법,” published by the NIKL.
If you’re to append the particle “을,” the object marker, you write “Cat을” or “캣을” and read it as “캣을” (/캐슬/) with the /t/ sound disappeared, because the Korean phonology applies as if it’s in Hangul.
—or a placeholder symbol?
Which particle to use for a blank usually doesn’t matter.
Write both variants of the particle
- “○은(는)” or “○은/는”
- “○이(가)” or “○이/가”
- “○을(를)” or “○을/를”
- “○와(과)” or “○와/과”
- “○(이)다” (or a bit rarely, “○이다/다”)
- “○(이)면” (or a bit rarely, “○이면/면”)
- “○(이)라서” (or a bit rarely, “○이라서/라서”)
- “○(이)라면” (or a bit rarely, “○이라면/라면”)
- “○(이)고” (or a bit rarely, “○이고/고”)
- “○이었다/였다” (You can’t group them by “었다;” “○(이)었다” doesn’t work.)
- “○이에요/예요” (You can’t group them by “에요;” “○(이)에요” doesn’t work.)
- “○아/야” or “○아(야)”
or just one of them, depending on how you’d like to read the blank ○.
- “○을”
- “○를”
- “○이”
- “○가”
- “○이다”
- “○다”
How you read the blank is up to you.
- “뭐뭐so-and-so를”
- “무엇무엇something-something을”
- “아무개Mr. or Ms. so-and-so가”
- “누구누구somebody가”
- “무엇무엇something-something이”
- or even literally, “빈칸blank이다,” “동그라미circle가,” or “네모rectangle였다,” because nobody cares.

But if it is supposed to be replaced with an actual word, you should have the following particle agree with that. Though you could also go for the easy and lazy ways listing both variants (the most common way for applications) or sticking to one of the variants, it’ll look either robotic or incorrect. Just like “a(n) [PLACEHOLDER]” in English.

Doing this right is actually a pretty tricky, notorious challenge for programmers, known as “조사 처리dealing with particles.” They even write a whole library just to handle this. What makes it so difficult to do that correctly is the transliteration of non-Korean words, where the application has to know how to read them.
On top of that, just because a word ends with a consonant doesn’t mean it guarantees a final consonant in its Korean transliteration. “Wisp” becomes “위습with a final,” “cats” becomes “캐츠without it,” “beer” becomes “비어without it!” Wow, how consistent!

Sophisticated applications would handle it properly by either
integrating such logic into themselves,
- “크레이지D(→크레이지디)는 무엇을 할까?” (“What will 크레이지D do?”)

or carefully phrasing template sentences in a way that doesn’t require it.
- “모험가Adventurer!”
- “[PLAYERNAME] 님-nim…….”
- “[PLAYERNAME],comma ……!”
