This is the 48th most frequent German word.
seit
"Seit" means "since" or "for" in English.
The word "seit" here is used as a preposition and indicates the starting point of an action in time that continues up to now.
Seit einem Jahr lerne ich Deutsch.
I have been learning German for a year.
The word "seit" functions as an adverb in this sentence, indicating the starting time of an ongoing situation.
Seit gestern hat es nicht aufgehört zu regnen.
Since yesterday, it has not stopped raining.
The word "seit" here introduces a subordinate clause and denotes the point in time when the action started in the past.