Because of the accusative. So we have our sentence which has our subject/object/verb. Linguistically, these can be "marked" in a language in many ways. In this language there are a couple ways that the information is conveyed.
So it follows an S-O-V style sentence structure, but it is adaptable. However it also uses noun "declensions" to mark it. There are many noun cases, but the two to point out here are the "nominative" and the "accusative" which mark the subject and the object respectively.
So in this sentence "Ābre urnen.", we have two words. One is "Ābre", declined in the accusative. This means that the woman is what is being acted upon in the sentence. The other "urnen", is the verb "to see" conjugated with the subject "I".
This is how we know that "I see the woman."
Here is the declension list that applies to "Ābra", it is labeled "Lunar-type: vala" as the model is the same for these words: