Subject and object
The subject is the person or the thing doing the action of the verb. The object is the person or the thing receiving it.
Juan compra una manzana (an apple). Subject: Juan. Object: una manzana.
Laura ama a Juan. Subject: Laura. Object: Juan.
Toco el piano. Subject: Yo (implied). Object: el piano.
María llama a Jorge. Subject: María. Object: Jorge.
Progress, Vol. 1
Progress, all volumes
Interleaving
Drilling one topic until it feels easy is tempting, but mixing different topics together builds more flexible, durable knowledge. The variety forces your brain to work harder, and that's what makes it last.