dahin
there
Indicates movement to a particular, often distant or not precisely specified place.
Der Weg führt uns direkt dahin, zum Gipfel des Berges.
The path leads us directly there, to the summit of the mountain.

there / gone
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there
Indicates movement to a particular, often distant or not precisely specified place.
Der Weg führt uns direkt dahin, zum Gipfel des Berges.
The path leads us directly there, to the summit of the mountain.

The word is made up of 'da' (there, at that place) and 'hin' (movement in a direction). So it literally describes movement toward a place.
Der Bus fährt bis zum Marktplatz, aber wir müssen noch weiter dahin.
The bus goes as far as the market square, but we have to go farther there.
In the meaning of 'lost' or 'over', 'dahin' (gone) is always combined with the verb 'sein' (to be). The structure is: [Something] ist/war dahin.
Nach dem Streit war die gute Stimmung dahin.
After the argument, the good mood was gone.
The combination 'bis dahin' (until then) is a fixed time expression and means 'until that point in time'. It can stand at the beginning of a sentence or in the middle of a sentence.
Wir sehen uns nächste Woche. Bis dahin wünsche ich dir alles Gute!
We'll see each other next week. Until then, I wish you all the best!
To describe an aimless, slow action, 'dahin' is often combined with verbs like 'leben' (to live), 'plätschern' (to babble), 'dösen' (to doze), or 'siechen' (to waste away). It often appears in the expression 'vor sich hin...'.
Der alte Mann saß auf der Bank und döste vor sich dahin.
The old man sat on the bench and dozed away.
there / since / then
'da' and 'dahin' both refer to a place, but they do not show the same thing. 'da' describes where something is: at this place. 'dahin' describes a direction or movement to this place. They are easy to confuse because both can point to a mentioned or known place. A memory aid: 'da' is the place, 'dahin' is the way to that place.

Zum Bahnhof fahre ich morgen früh dahin.
I'm going there, to the train station, early tomorrow morning.
Deine Tasche liegt da neben der Tür.
Your bag is there next to the door.