Korean Language for Beginners

이/가 AS A SUBJECT MARKER

One of the most difficult things for a new learner of Korean to understand is the difference between the particles 는/은 and 이/가. You learned that 이/가 on the object that has a person "has" when using 있다. In addition to this, there are more functions of 이/가 that you should know about. You learned that you should add 는/은 to the subject of the sentence. For example:
고양이는 집 뒤에 있다 The cat is behind the house

In this sentence, notice that the particle 는/은 indicates that the "cat" is the subject. However, the sentence above could also be written like this:
고양이가 집 뒤에 있다 The cat is behind the house

The two sentences could have exactly the same meaning and feeling. I emphasise "could" because in some situations, the meaning of the two sentences is exactly the same but in other situations the meaning of two sentences can be subtly different.

The reason why they could be identical :
고양이는 집 뒤에 있다 The cat is behind the house
고양이가 집 뒤에 있다 The cat is behind the house

이/가, like 는/은 is added to the subject of the sentence. In some situations, there is no difference in meaning or feeling between adding 이/가 or 는/은 to the subject.

The reason why they could be subtly different :
는/은 has a role of indicating that something is being compared with something else. The noun that 는/은 is added to is being compared. For example:
고양이는 집 뒤에 있다 The cat is behind the house

The speaker is saying that the cat is behind the house (in comparison to something else that is not behind the house). The difficulty here is that there is only one sentence which gives the listener no context to understand what the "cat" is being compared with. However, if I were to make up a context that fits into this situation, it could be that "The dog is inside the house and the cat is behind the house". However, saying:
고양이가 집 뒤에 있다 The cat is behind the house

... is simply stating a fact and the "cat" is not being compared to anything.

From the example above (using 는/은 or 이/가), the translation does not change by altering the subject particle. Rather, the only thing that changes is the subtle feeling or nuance that something is being compared. Note that this "comparative" function of 는/은 can be used in much more complicated sentences and can be attached to other grammatical principles.

The good thing is, even if you make a mistake with the usages of 이/가 and 는/은, 99.9% of the time, the listener will be able to understand exactly what you are trying to express. Likewise, if you listen to somebody speaking, you will be able to understand what they are trying to say regardless of if you have learned the more complex usages of 이/가 and 는/은. The difference between these two particles is about nuance and does not dramatically change the meaning of the sentence. Making a mistake between other particles, however, would cause other people to misunderstand you. For example, using 를/을 instead of 는/은 would (most likely) make your sentence incomprehensible.