In English, the present progressive is expressed by adding -ing. In Korean, the gerund is expressed by the construction ~고 있다.

변호사들이 서류를 준비하고 있어요 = The lawyers are preparing the documents

(변호사 = lawyer. 들 = plural marker. 이 = subject marker. 서류 = document. 를 = object marker. 준비하고 있어요 = preparing)

The verb used in the above sentence was 준비하다 = to prepare. To form the present progressive, drop the 다 of 준비하다 and add it to 고 있다. 있다 was conjugated in the standard polite in the example above, but it can also be conjugated in the deferential polite as well.

With the sentence below, can you tell how this construction is different from 고 있다?

유리는 책을 읽고 있었어요 = Yuri was reading a book.

(유리 = Yuri. 는 = topic marker. 책 = book. 을 = object marker. 읽고 있었어요 = was reading)

The construction ~고 있었다 was used instead of ~고 있다. With the ~고 있었다  construction, the -ing remains intact, but unlike ~고 있다, the action is happening in the past. As a result, if the above construction was replaced by ~고 있다, it would mean that Yuri “is” reading a book, instead of “was” reading a book. Also, as a sidenote, 있었다 was conjugated as 있었어요 in the standard polite, but the deferential polite can also be used.

There’s another construction that is similar in meaning to 고 있다, but with a specific nuance that is completely different from 고 있다.

그 행사가 범죄를 조사하는 중이에요 = That detective is in the middle of investigating the crime

(그 = that. 행사 = detective. 가 = subject marker. 범죄 = crime. 를 = object marker. 조사하는 중이에요 = in the middle of investigating)

The construction used was 은/는 중이다, which means, “in the middle of” or “in the process of”

If you take the same sentence above and replace it with 고 있다, it would mean “That detective is investigating the crime”.