I don't want to sound like I am talking about grammar ALL DAY, but it seems like I do already, so I'm gonna write anyway: a short thread about the simplest and the most basic particles (or postpositions?) in Korean.

If you are done with studying the most basic Korean, you've most likely encountered 은,는/을,를/이,가. But these are also what people have most errors about. This is a tip: think about these 2 questions when you revise your sentence. I can't make it simpler.

1) is it a subject or direct object? 2) do you want to emphasize it?

If it is a subject, then it is either 이,가 or 은/는.
If it is a direct object, then it is either 을/를 or 은/는.
If you wanna emphasize it, it is 은/는.

I will try to explain it without using any Korean terms lol. I obviously wasnt taught Korean in English so it could differ from conventional terms.

이/가 are subject markers. 을/를 are object markers. 은/는 are topic markers (regardless of its use in the sentence.)

The first ones (이/을/은) are used if the word coming before it ends with a consonant, and the latter ones (가/를/는) are used if vowel comes before it.

There is a simple quiz for anyone reading, before you go on to the next tweet:

엄마[ ] 선풍기[ ] 켰다. (Mom turned on a fan.)
눈[ ] 왔다. (It snowed.)
아버님[ ] 쓰레기통[ ] 비웠다. (Father emptied the trashcan.)

Which of the mentioned particles should be in the blanks?

Continuing:
은/는 are topic marker and quite literally, they mark the topic. Wait, we already have the subject marker! Right? But subject is not always the topic. It is the agent. You use 은/는 to emphasize the topic, and it might be subject, in/direct object,or even verb.

Consider the following example:

누가 밥을 먹었니? Who ate?
내가 먹었어. I did.

네 여동생은 밥 먹었대. 너는 먹었니?
I heard your sister ate. Did YOU?

너희끼리 밥 먹어. 나는 이미 먹었어.
You guys eat, I already have.

It can be attached to different part of the sentence, and I'm gonna use the same sentence for each of it. Notice that there is no big difference in meaning but only in the emphasis, but in this case it also changes the meaning. Consider:

내가 밥을 먹었다. I ate.

I didn't translate the object because 밥을 먹다 is as a whole thought as "eating" and not only eating rice. (Well... rice is always there when we're eating...)

Also consider:
나는 밥을 먹었다. "I" ate. (Because there is no context here and I is subject anyway, there is no feeling of emphasis, really.)
내가/나는 밥은 먹었다. I ate RICE. (Feels like I ate rice but nothing else.)
나는 밥을 먹기는 했다. I ATE rice. (Feels like-

I ate but didn't do anything else, like washing dishes for example, or maybe I ate but wasn't satisfied?)

So 은/는 can be attached to many different things.

Not the topic of this thread but -에게 (to someone) can also use 은/는. Like in:
나 그애한테는 초콜릿 줬는데 너한테는 안 줬어. I gave him/her/them a chocolate but not to you.

Note that -에게 can be omitted here:
다른 사람 다 줘도 너(에게)는 안 줘.
I can give everyone but YOU.