- "Depuis" is equivalent of the english word since. Use it to express the duration of an event that still lasts:
- Pierre travaille dans un musée depuis six ans.
- Depuis ce matin j'attends le livreur.
- "Dès" is usually translated as the english phrase as soon as. Use it to express a starting point of an event with some urgency:
- Je vous appelle dès la réception du colis.
- Dès demain, il arrête de fumer.
- "À partir de" is equivalent of the english word from. Use it to express a starting point of an event without urgency:
- À partir du 1er janvier il commence un nouveau travail.
- Nous sommes disponibles à partir de maintenant.
Gardien de musée
We use "depuis" when event started in past. Such event still lasts at the moment when one speaks.
That means that Pierre works since 2015. But, that also means that he still works there. Therefore "depuis" is used with present tense of the verb.
We also use "depuis" to express the duration that still lasts.
We use "dès" to show an immediate starting point.
The speaker wants to give the impression of some urgency.
He started to look for the director immediately.
It does not matter if it still lasts or not.
"Depuis" insists on the idea of duration of an event which continues until now.
It lasts, he still haven't found it.
"À partir de"
We use "à partir de" to reference a starting point. In this case it's the synonym to "dès". However, "à partir de" is more neutral. There is not the same sense of urgency. And there is neither the idea of duration like with the word "depuis".