der gute Freund

Lesson 15 covered adjectives after ein, where the adjective has to carry the gender itself — ein guter Freund. This lesson does the other half, and it is genuinely easier, because der/die/das already shows the gender. The adjective has almost no work left to do.

After a definite article there are only two endings in the whole A1 range: -e and -en.

Here is the entire pattern. Do not memorise it as a grid — read it once, then get the shape from the sentences underneath.

Now the shape, which is what you actually need. The top-left corner — singular, nominative and accusative — is -e, with one exception: masculine accusative is -en. Everything else, all of it, is -en.

Three things that are always -en: masculine accusative, the whole dative, and the whole plural.

This is the same masculine-accusative move you have been making since Lesson 3. Der Mann → den Mann; der alte Mann → den alten Mann. The article changes and the adjective follows it.

• Der alte Mann wohnt hier.

  The old man lives here.

  masculine subject → -e

• Ich sehe den alten Mann.

  I see the old man.

  masculine object → -en

• Das ist die neue Wohnung.

  That is the new flat.

• Ich fahre mit dem neuen Auto.

  I am driving the new car.

  dative → -en, always

• Die kleinen Kinder schlafen.

  The small children are asleep.

  plural → -en, always

• Ich spreche mit den guten Freunden.

  I am talking to the good friends.

  dative plural: -en on the adjective and -n on the noun

One thing to keep clear in your head: this lesson and Lesson 15 describe different situations, not competing rules. ein guter Freund but der gute Freund. Which one you use depends on the article in front, so decide the article first and the ending follows.

And possessives behave like ein: mein guter Freund, meinen guten Freund — except in the plural and dative, where they take -en like everything else: meine guten Freunde, mit meinen guten Freunden.