
erst
not until
Means 'not before a certain point in time' or 'only.'
Wir können erst um 18 Uhr gehen, weil wir vorher noch arbeiten müssen.
We can leave only at 6 p.m. because we still have to work before then.
only / not until / first
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not until
Means 'not before a certain point in time' or 'only.'
Wir können erst um 18 Uhr gehen, weil wir vorher noch arbeiten müssen.
We can leave only at 6 p.m. because we still have to work before then.

only recently
Indicates that something happened a short time ago; 'recently.'
Ich habe erst gestern davon erfahren.
I only found out about it yesterday.

first
Means 'first' or 'at the beginning' and introduces a sequence.
Erst war ich unsicher, aber dann hat mich die Idee überzeugt.
At first I was unsure, but then the idea convinced me.

only
Means 'only' in relation to an amount or number, often with the implication that it is not much yet.
Sie ist erst 16 Jahre alt und schon so erfolgreich.
She is only 16 years old and already so successful.
When 'erst' refers to time, it means 'not before' a certain point in time. The focus is on the earliest possible moment when something can or will happen.
Ich kann erst am Abend kommen, weil ich lange arbeiten muss.
I can only come in the evening because I have to work late.
When 'erst' is used with a number or an age, it means 'only,' but implies that the amount or age is considered low and is expected to increase. It has a dynamic, 'not yet completed' meaning.
Ich habe erst zwei Seiten des Buches gelesen, aber es ist schon spannend.
I have only read two pages of the book, but it is already exciting.
To describe a sequence of actions, 'erst' is often used for the first step. In a following sentence or clause, 'dann' (then) or 'danach' (after that) often appears to form the structure 'zuerst ..., dann ...' (first ..., then ...).
Erst räumen wir das Zimmer auf, dann gehen wir Eis essen.
First we tidy up the room, then we go eat ice cream.
In the meaning 'vor Kurzem' (recently), 'erst' is often combined with a time expression referring to the recent past, for example 'gestern' (yesterday), 'letzte Woche' (last week), or 'vor ein paar Tagen' (a few days ago). It emphasizes how short the elapsed time is.
Wir sind erst letzte Woche aus dem Urlaub zurückgekommen.
We only got back from vacation last week.
only
Nur drei limits the number to three. Erst drei presents the same number as the current, still fairly low total and usually suggests that more will be added later. Without this expectation of development, nur is the better choice.

Wir haben erst drei Stühle gesammelt und brauchen noch weitere.
We've only collected three chairs so far and still need more.
Wir brauchen nur drei Stühle für unsere Gäste.
We only need three chairs for our guests.