This is the 47th most frequent Danish word.
siden
"Siden" in Danish means "since" or "ago," depending on the context.
In this sentence, 'siden' denotes a point in time (last summer) from which an action or state continues to the present moment.
Jeg har ikke set hende siden sidste sommer.
I haven't seen her since last summer.
Here, 'siden' is used to indicate a cause or reason for an action, similar to 'because' or 'since' in English.
Siden du lagde mærke til regnen, blev vi derhjemme.
Since you noticed the rain, we stayed home.
In this case, 'siden' acts as an adverb referring to a previously mentioned point in time, without repeating the time phrase explicitly.