since
The German word 'seit' is used to express the concept of 'since' in English. It is often used to indicate a point in time or a duration from which something has been happening. For example, 'Ich wohne seit fünf Jahren in Berlin' translates to 'I have been living in Berlin for five years.' However, unlike 'since' in English, 'seit' is not used to introduce reasons or causes.
Since when are you learning German?
This is a question asking about the duration someone has been learning German. 'Seit' in this context is used to indicate a starting point in the past until now.
I have been living in Berlin for five years.
This is a statement expressing the duration of the speaker's residence in Berlin. 'Seit' is used to indicate a starting point in the past lasting up until now.
He has been working here since Monday.
This is a statement describing how long someone has been working somewhere. 'Seit' indicates a starting point in the past that continues up to the present moment.