Ask Wem?
Find the Dativ object.

One question. One answer. That answer is always Dativ. The Dativ has a question word built for it. Use it and the recipient surfaces every time.

Related: The German Dativ Case, the full picture of the recipient case.

Wem? means: who is receiving it?

Wem means "to whom." You point it at the verb and ask: who is the action going to? Who is getting something? The word that answers is your Dativ. It is the person the action reaches. In edupango, it is always blue.

Three sentences. Same question every time:

• Er danktdemLehrer. Wem dankt er? → demLehrer.
• Sie vertrautihrer Schwester. Wem vertraut sie? → ihrer Schwester.
• Ich helfemeinem Freund. Wem helfe ich? → meinem Freund.

In every case, the answer is blue. Something is moving toward it. The action has a destination.

The Dativ is the recipient

The Nominativ does the action. The Akkusativ receives the action directly. The Dativ is the one the action goes to. It is a step removed. The book moves from the subject to the Akkusativ, but the person who gets it is always Dativ.

Two nouns in one sentence:

• DerLehrer erklärtdemSchülerdieRegel.

Ask Wen? and you find the direct object. Ask Wem? and you find the recipient. Two questions, two answers, two colors.

In a sentence like that one the Dativ really is the indirect object, because a direct object sits next to it. But many verbs take a Dativ and nothing else: helfen, danken, vertrauen, gefallen. Ich helfedemMann. has no direct object at all, so demMann is simply a Dativ object. The question finds it either way.

The articles change more here

The Dativ is the case where articles change the most. Every gender shifts.

Gender	Nominativ	Dativ
Masculine	derMann	demMann
Feminine	dieFrau	derFrau
Neuter	dasKind	demKind
Plural	dieLeute	denLeuten

The pattern is worth learning. dem covers masculine and neuter. der takes feminine. And Dativ plural always gets den. The noun gets an extra -n too, unless it already ends in -n or -s → denFrauen and denAutos stay unchanged.