
längst
long ago / already
Expresses that something has already been the case for a long time or happened a long time ago.
Als ich ankam, waren die anderen längst gegangen.
When I arrived, the others had long since left.
long ago / by now
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long ago / already
Expresses that something has already been the case for a long time or happened a long time ago.
Als ich ankam, waren die anderen längst gegangen.
When I arrived, the others had long since left.

by no means
In the phrase 'längst nicht,' it means 'by no means' or 'nowhere near' and strongly weakens a statement.
Das Projekt ist längst nicht fertig, es gibt noch viel zu tun.
The project is nowhere near finished; there is still a lot to do.
'Längst' is a form of the superlative of 'lang' (long). It expresses that something has been this way, or was this way, for a very long time already.
Diese Technologie ist längst veraltet.
This technology has long been obsolete.
The meaning of 'längst' changes completely with 'nicht.' On its own, 'längst' means 'already for a long time,' but in the combination 'längst nicht,' it means 'nowhere near' or 'by no means.'
Er ist längst fertig mit seiner Arbeit.
He has long since finished his work.
'Längst' often has a subjective nuance and expresses surprise, impatience, or reproach. It emphasizes that something happened much earlier or should have happened much earlier.
Du kommst jetzt erst? Ich warte schon längst auf dich!
You're only coming now? I've been waiting for you for ages!
The combination 'längst nicht' is a fixed expression that strongly weakens or negates a statement. It is often used to emphasize that a state has not yet been reached.
Obwohl wir schon drei Stunden arbeiten, sind wir längst nicht fertig.
Although we've already been working for three hours, we are nowhere near finished.
already
Schon neutrally indicates that something has already happened, often earlier than expected. Längst, on the other hand, emphasizes that it happened a long time ago or has been the case for a long time. When the long time span is important, längst is more appropriate.

Mia hat den Bericht längst abgegeben.
Mia submitted the report a long time ago.
Mia hat den Bericht schon abgegeben.
Mia has already submitted the report.