Nominativ.
Who is doing the thing?

The Nominativ is the subject. That's the whole lesson. Everything else is detail.

Also: Once you have the Nominativ, the next step is the Akkusativ. See German Akkusativ Case. And for the pronouns that always live here: Personal pronouns. The Nominativ set.

The question that finds the subject

Before the verb, ask → Wer? oder was? "Who or what?" Whatever answers that question is Nominativ. It's the person or thing driving the sentence. In edupango, it's always Nominativ.

Three sentences. Same question, same color every time:

• DerHund bellt. Who is barking? → derHund. → Nominativ.
• DieLehrerin erklärt. Who is explaining? → dieLehrerin. → Nominativ.
• DasKind schläft. Who is sleeping? → dasKind. → Nominativ.

You don't need to think about the ending. You ask the question, you find the Nominativ. That's the system.

The articles: already familiar

The Nominativ articles are the forms you learned with every noun. No changes yet. This is home base.

Gender	Definite (the)	Indefinite (a)
Masculine	derMann	einMann
Feminine	dieFrau	eineFrau
Neuter	dasKind	einKind
Plural	dieLeute	-

Every other case is a departure from these. Learn to feel home base first, and every move away from it becomes a move you can recognize.