Learn Hardcore German: Der Trainer erklärte, dass dieser Sieg nur der Anfang sei. - The coach explained that this victory was only the beginning.

dass is a subordinating conjunction that introduces the content clause (Inhaltssatz) of erklärte (“explained that…”). As a result, everything that follows up to the end of that clause belongs to a subordinate clause, and the finite verb (sei) is sent to the very end.

Why does the verb sein appear as sei instead of ist?

sei is the Konjunktiv I form of sein, used to mark indirect speech (reported speech). By using Konjunktiv I, German signals that these are the trainer’s words, not a direct assertion by the narrator. If you used ist, it would sound like you’re affirming it as your own statement rather than reporting.

Why is the verb at the end of the subordinate clause?

In German, subordinate clauses introduced by conjunctions like dass follow the “verb-final” rule: the finite verb (here sei) must appear in the last position of that clause.

What tense is erklärte? Could we use hat erklärt instead?

erklärte is the simple past (Präteritum). In written narratives and formal registers, Präteritum is often preferred. In spoken or informal German you might say hat erklärt, which is the Perfekt – both are grammatically correct.

What is the function of dieser in dieser Sieg? Could we just say der Sieg?

dieser is a demonstrative determiner (“this”). It emphasizes the immediate victory just achieved. You could say der Sieg (“the victory”), but dieser Sieg underlines “this very victory” and often contrasts it with future successes.

Why are dieser Sieg and der Anfang in the nominative case?

Within the dass-clause:

dieser Sieg is the subject (Nominativ).
der Anfang is a predicate nominative (also Nominativ) linked by the copula sei.
Predicate nominatives always share the case of the subject.

Why is nur placed before der Anfang?

nur is an adverb restricting the noun phrase that follows. In German, limiting adverbs typically precede the element they modify—in this case der Anfang—to indicate “only the beginning.”

How would you rewrite this sentence in direct speech?

You could say:
Der Trainer erklärte: “Dieser Sieg ist nur der Anfang.”