This is the 58th most frequent Portuguese word.
desde
"Desde" means "since" or "from" in English, indicating a starting point in time or place.
Here, 'desde' is used as a preposition indicating the point in time when the action (living) started.
Eu moro nesta cidade desde 2010.
I have lived in this city since 2010.
'Desde' in this example is used as a preposition indicating the reference point in time for the change described (him being different).
Ele está diferente desde sua última viagem.
He has been different since his last trip.
In this sentence, 'desde que' introduces a temporal clause specifying the start of the condition (starting to study).