Prepositions connect words and groups of words together. They express the relation between living beings, things or contexts.
Prepositions that describe relations of time are called temporal prepositions. You're already familiar with some of them:
"im Sommer" (in the summer), "am Montag" (on Monday), "um 14 Uhr" (at 2 o'clock).
The following temporal prepositions describe a duration of time. In other words, they indicate a longer interval. Each of them expresses something different about the beginning and end of that period of time.
| seit | describes a period of time that began in the past and continues up to now. | Nick schreibt seit 11 Jahren Artikel über Physik. (= Nick began writing articles about physics 11 years ago. He still does so today.) |
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| von … bis | specifies the beginning and end of a period of time. That period of time can be in the past or in the future. It can start now or in the future. | Von 2005 bis 2008 forschte er an der Universität München. (= Nick began conducting research at the University of Munich in 2005 and stopped in 2008.) |
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| ab | describes the beginning of a period of time starting now or in the future. The end is indefinite. | Ab dem kommenden Semester ist Nick Physikprofessor an der Universität Hamburg. (= Nick will begin his job as a physics professor next semester. There is no indication of when he will stop.) |
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| bis | indicates the end of a period of time but says nothing about its beginning. | Wir brauchen den Artikel über die Zeitschleife bis Freitag. (= It doesn't matter when Nick begins writing his article, but he needs to hand it in by Friday at the latest.) |
More:
To ask about a duration of time, you simply place the question word "Wann?" in front of the corresponding preposition.
Examples:
Question: Seit wann schreibt Nick Artikel über Physik?
Answer: Nick schreibt seit 11 Jahren Artikel über Physik.
Question: Von wann bis wann forschte Nick an der Universität München?
Answer: Von 2005 bis 2008 forschte er an der Universität München.
Question: Ab wann ist Nick Physikprofessor an der Universität Hamburg?
Answer: Ab dem kommenden Semester ist Nick Physikprofessor an der Universität Hamburg.
Question: Bis wann brauchen Sie den Artikel über die Zeitschleife?
Answer: Wir brauchen den Artikel über die Zeitschleife bis Freitag.