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.

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' and 'erst' can both limit an amount or a point in time. 'nur' is more neutral: it simply says that it is not more than this amount or thing. 'erst' often also means 'only', but with an expectation: it is still early, still not much, or something will come later. That is why 'erst' can easily suggest a development. Learners confuse the words especially with numbers and times. With amounts, 'nur' is a pure limitation; 'erst' often sounds as if more will come later.

Die Ärztin kommt erst um zehn Uhr in die Praxis.
The doctor will not come to the practice until ten o'clock.
Für das Wochenende haben wir nur zwei kleine Aufgaben geplant.
For the weekend, we have planned only two small tasks.