Hi @allen10086
Great question! You're absolutely right that eins functions as a direct object pronoun here, and German word order rules can be tricky.
In general, German follows the TeKaMoLo (Temporal - Kausal - Modal - Lokal) structure for adverbial phrases, but pronouns tend to take precedence. When you have a direct object pronoun (eins), it typically comes before time expressions like morgen. This is why "Du musst morgen eins kaufen" is correct rather than "Du musst eins morgen kaufen".
This is a special case where the presence of a pronoun slightly overrides the usual time-first rule. Keep up the great work, and let us know if you have any other questions!