This is the 724th most frequent German word.
erster
"Erster" means "first" in English.
In this sentence, 'erste' is used as an adjective describing the position in an order or sequence.
Ich war der erste, der den Raum betreten hat.
I was the first to enter the room.
In this sentence, 'ersten' is the dative form of 'erste,' used because it follows the preposition 'am' combined with 'Tag' in the dative case.
Am ersten Tag der Woche sind wir spazieren gegangen.
On the first day of the week, we went for a walk.
Here, 'erste' describes 'Eindruck,' serving as an adjective to specify the sequence or priority of impressions.