Zeitpräpositionen: um, seit, vor, ab, bis

German uses a different preposition for each kind of time expression:

• um + clock time: um acht Uhr (at eight o'clock)
• von … bis for a period with a start and an end: von Montag bis Freitag (from Monday to Friday)
• bis alone = until: bis Sonntag (until Sunday)
• seit = since / for, for something that started in the past and is still going on. It goes with the present tense: Ich wohne seit drei Jahren hier. (I have lived here for three years.)
• vor = ago, with a past tense: vor zwei Tagen (two days ago)
• ab = from … on, starting at a point, usually in the future: ab morgen (from tomorrow)
• in = in, counted from now: in einer Woche (in a week)
• nach = after an event: nach dem Essen (after the meal)

Tip: the tense of the verb often tells you whether you need seit, vor or in.