fort
away
No longer present in a place; gone.
Als ich ankam, war er leider schon fort.
When I arrived, unfortunately he was already gone.

away / forth
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away
No longer present in a place; gone.
Als ich ankam, war er leider schon fort.
When I arrived, unfortunately he was already gone.

'Fort-' is a common separable prefix. It usually means 'weiter' (on/continue), as in 'fortsetzen' (to continue), or 'weg' (away), as in 'fortgehen' (to go away). Recognizing this prefix helps you understand many verbs.
Wir setzen die Reise morgen früh fort.
We will continue the journey tomorrow morning.
In the meaning 'weg' (away), 'fort' is very often used with the verb 'sein' (to be). It then stands at the end of the sentence and describes a state.
Als wir zurückkamen, waren die Vögel fort.
When we came back, the birds were gone.
'Fort' is part of two important fixed expressions: 'und so fort' (usf.) means 'and so on.' 'In einem fort' means 'without interruption' or 'constantly.'
Im Supermarkt kaufe ich Obst, Gemüse, Brot und so fort.
At the supermarket I buy fruit, vegetables, bread, and so on.
In the meaning 'weg' (away), 'fort' is frequently used in written language or in a somewhat elevated style. It can sound somewhat more formal or literary.
Der Sommer war nun endgültig fort.
The summer was now finally gone.
there / since / then
'da' and 'fort' can form a clear contrast when it comes to presence. 'da' then means that someone or something is present. 'fort' means that someone or something is no longer there, so they are gone. 'da' sounds very normal in everyday language. 'fort' sounds somewhat more elevated or written than 'weg' (away / gone), but it is easy to understand in this contrast.

Als ich nach Hause kam, war der Gast schon fort.
When I came home, the guest was already gone.
Der Mitarbeiter ist heute da und kann die Frage beantworten.
The employee is here today and can answer the question.