The object is accusative, but the visible sign is not always a fatḥah. These 4 forms already appear in this level's sentences.
Object form
Accusative sign seen here
Example
singular noun
fatḥah
الْأَرْضَ
dual noun
ياء
الْوَلَدَيْنِ
sound masculine plural
ياء
الْكَافِرِينَ
sound feminine plural
kasrah
السَّمَاوَاتِ