Sophia says: Find the subject and object
You will have noticed that it makes a big difference whether you do something or have it done to you.
The person (or thing) doing something in a sentence is the subject in the nominative case. The person (or thing) having something done to them is the object in the accusative case. This is a simplified summary, but it will do for now.
Consider this example:
‛ο Θεοδωρος λυει την Σοφιαν, αλλα ‛η Σοφια φευγει.
In the first part of the sentence Sophia is the object in the accusative case, but in the second part of the sentence she is the subject so nominative. In Greek, the article as well as the noun ending will often help you to recognise the case.