mit einem guten Freund

You already know two thirds of this. Lesson 15 gave you ein guter Freund and einen guten Freund; Lesson 20 gave you the whole definite-article system. What is missing is the dative and the plural after ein-words — and those are the two easiest parts, because they are almost all -en.

An "ein-word" is ein, kein, and every possessive: mein, dein, sein, ihr, unser, euer, Ihr. They all take exactly the same adjective endings.

That is the real payoff of this lesson. Learn it once for ein and you have it for mein, kein, ihr and the rest at no extra cost.

Start with what you know, then read the new row.

The entire dative row is -en. The entire plural column is -en. No exceptions.

So the only endings you ever have to think about are in the top-left block — and you learned those a while ago. The three "hard" endings are the ones where the article itself gives nothing away: -er (masculine nominative), -es (neuter), and masculine accusative -en.

There is a reason behind it, and it is worth one sentence: ein is the same word for masculine and neuter, so it cannot show gender. The adjective steps in and carries the ending the article is missing — einer Freund is impossible, so it becomes ein guter Freund. After dem, der, den the article has already done the job, so the adjective relaxes into -en.

• Das ist ein guter Freund.

  That is a good friend.

  masculine subject — the adjective carries the -er

• Ich habe einen guten Freund.

  I have a good friend.

  accusative object

• Ich spreche mit einem guten Freund.

  I am speaking with a good friend.

  mit → dative → -en

• Wir haben eine neue Wohnung.

  We have a new flat.

• Ich wohne in einer neuen Wohnung.

  I live in a new flat.

  in + wo? → dative → einer neuen

• Ich habe keine guten Freunde in Berlin.

  I have no good friends in Berlin.

  plural → -en, whatever the case

• Ich fahre mit keinen guten Schuhen.

  I am travelling with no good shoes.

  dative plural: -en on the article, on the adjective, and on the noun

The dative plural is the one place where three -n appear in a row: mit keinen guten Freunden. If you can produce that, the rest of the system holds no surprises.

Choosing the case is the real skill, and you already have it. Ask the same questions as always: is it the subject (nominative), the object (accusative), or is there a dative trigger — mit, bei, von, zu, seit, aus, nach, or in/auf/an answering wo?

• Mein neuer Chef ist sehr nett.

  My new boss is very nice.

  subject → nominative → -er

• Ich kenne meinen neuen Chef noch nicht.

  I do not know my new boss yet.

  object → accusative → -en

• Ich arbeite mit meinem neuen Chef.

  I work with my new boss.

  mit → dative → -en