Questions & Answers about Sie ist seit Mittwoch krank.
Why does the German sentence use ist (present tense) when the English translation says "has been"?
In German, if something started in the past and is still happening right now, you use the present tense with seit. English uses the present perfect ("has been"). Because she is still sick, we say Sie ist seit Mittwoch krank.
Can seit mean both "since" and "for"? How do I know which one it is here?
Yes! seit translates to "since" when followed by a specific starting point in time, like Mittwoch (Wednesday). It translates to "for" when followed by a duration of time, like seit drei Tagen (for three days).
I learned that seit requires the dative case. Why doesn't Mittwoch change to show the dative?
When using days of the week or months directly after seit, we usually drop the article (the word for "the"). Without the article dem, the dative case is "invisible" here, so it just looks like the regular dictionary word Mittwoch.
Could I use von Mittwoch bis heute (from Wednesday until today) instead of seit Mittwoch?
You could, and people would understand you, but seit is much more natural and common when a state is still ongoing. You generally use von ... bis when both the start and end times are clearly defined limits.