Learn Hardcore German: Ich bemerkte den Fehler erst, nachdem wir den Bericht abgeschickt hatten. - I noticed the mistake only after we had sent the report.

Erst here means only (indicating that the noticing happened no earlier than after the report was sent). In German, erst often appears directly before the time expression or subordinate clause it modifies. Placing it before the comma connects it to the entire following clause: “I noticed the mistake only after we had sent the report.”

Why is nachdem used here, and how does it affect word order?

Nachdem is a subordinating conjunction meaning after (marking that one past action follows another). In German subordinate clauses introduced by words like nachdem, the finite verb moves to the very end. That’s why you get nachdem wir den Bericht abgeschickt hatten, with hatten (the auxiliary) at the end.

Why is the subordinate clause in Plusquamperfekt (hatten abgeschickt) rather than in simple past or Perfekt?

Because the action of sending the report happened before the action of noticing the mistake. German uses Plusquamperfekt (past perfect) for an event that preceded another past event. The main clause is in Präteritum (bemerkte) and the earlier action is in Plusquamperfekt (hatten abgeschickt) to make the time sequence crystal clear.

Could I say nachdem wir den Bericht abgeschickt haben (Perfekt) instead of hatten abgeschickt?

In colloquial speech you might hear nachdem wir den Bericht abgeschickt haben, but standard grammar advises using Plusquamperfekt after nachdem when both events are in the past. Using Perfekt there can make the sequence of events less precise.

What’s the difference between als and nachdem, and why can’t I use als here?

Als is used for a single point or period in the past and translates to when (“when I was a child…”). Nachdem translates to after and emphasizes sequence (first A happened, then B). Since you want to express “after we had sent the report,” only nachdem is appropriate.

Why is den Fehler in the accusative case?

Fehler is the direct object of bemerkte (“to notice something”). In German, direct objects take the accusative case. The masculine accusative article is den, so der Fehler becomes den Fehler.

In the subordinate clause, why does abgeschickt stay together as one word instead of splitting into ab and geschickt?

When you form Perfekt or Plusquamperfekt with a separable verb like abschicken, you combine its prefix and participle into one word: abgeschickt. The separable split only occurs in simple tenses in main clauses (e.g., Ich schicke den Bericht ab).

Why is there a comma before nachdem?

German orthography requires a comma to separate a main clause from a subordinate clause introduced by conjunctions such as nachdem, weil, dass, etc. Always place a comma before these subordinators.

What happens to the word order if the nachdem-clause comes first in the sentence?

If the subordinate clause leads, you must invert subject and verb in the main clause that follows. For example:
Nachdem wir den Bericht abgeschickt hatten, bemerkte ich den Fehler erst.

Can I use Perfekt instead of Präteritum in the main clause (e.g., Ich habe den Fehler erst bemerkt…)?

Yes. In everyday spoken German and informal writing, people often use Perfekt:
Ich habe den Fehler erst bemerkt, nachdem wir den Bericht abgeschickt hatten.
In formal written narratives, though, Präteritum (bemerkte) is more common.