Is there any hint in the sentence that it is in past tense?
서다 -> 서는 = standing -> 선 = stood -> 설 = will stand
서다 is the written form or base form of 'to stand'. The modifier ㄴ indicates that it is past tense. 선 = Stood
선 is past tense. If it was 서 있는, it would be a present continuous, and if it was 서는, it can be translate into 'a horse that stand infront of the house'.
선, as opposed to 서는, if it's an action verb.
V + ㄴ indicates that the verb has been done. In this case 선.
선 is past tense. Present woulf be 서는.
Could i have said "who stood"..?