There are two types of adverb or adverbial phrase 1) sentence adverb(문장부사어) which modifies a whole sentence can be placed anywhere in the sentence e.g. 이상하게도 이상하게도 철수가 시험에서 떨어졌다. 철수가 이상하게도 시험에서 떨어졌다. 철수가 시험에서 이상하게도 떨어졌다. Strangely, Chulsoo failed the exam. 2) constituent adverb(성분부사어) which modifes a part of speech is usually placed before a modificand e.g. 무척 선생님은 철수를 무척 사랑하셨다. The teacher loved Chulsoo very much. When there are more than two adverbs in a sentence, The basic order for a Korean sentence is - 누가 언제 어디서 무엇을 왜/어떻게 [합니까] Who when where what why/how [does] The order of 왜 and 어떻게 is not that important - it doesn’t really matter whether you put 왜 on the 5th place and 어떻게 on the 6th or vice versa. e.g. 저는 점심에 식당에서 라면을 맛있게 먹습니다. I - in the afternoon - at a restaurant - ramen - deliciously - eat. I deliciously eat ramen at a restaurant in the afternoon. e.g. 앞으로 더 열심히 하겠습니다 하겠습니다 is predicate 앞으로(forward), 더(more), 열심히(hard) are adverbs 더 is before 열심히 because it emphasis 열심히. 앞으로 is when and 열심히 is how, so 앞으로 is before 열심히 In your example, 이미 is a constituent adverb(성분부사어) 저는 숙제를 이미 끝냈어요. is natural. in this case, 이미 modifies (끝내다) 저는 이미 숙제를 끝냈어요. is also possible. in this case, 이미 modifies (숙제를 끝내다)

ideal placement is right before the verb, but you'll see them all over the place. as you know Korean word order is very flexible so long as the particles are right and the verb comes last. I say anywhere after the subject/topic but before the verb is a legit placement