seit/seitdem

The words seit and seitdem connect a starting event to a subsequent ongoing situation in the present or past. The main clause describes the action that continues, while the subordinate clause can refer to either an ongoing action or a completed event.

1. Main Roles:

• As a Preposition: Seit (but never seitdem) is a dative preposition used to mean 'since' a point in time or 'for' an ongoing duration. Everything that follows it takes dative endings (e.g., 'seit einem Jahr', 'seit einer Woche', 'seit Monaten').
• As Conjunctions: Both can introduce a subordinate clause where the verb goes to the end. If the sentence starts with this subordinate clause, the following main clause must immediately begin with the verb to maintain verb-second word order (e.g., 'Seitdem ich in Berlin wohne, bin ich glücklich' - Since I have been living in Berlin, I have been happy).
• As Adverbs: Seitdem (and its synonym seither) often functions as a temporal adverb meaning 'since then'. It can occupy position 1, where the verb must follow in position 2 (e.g., 'Seitdem wohne ich in Berlin' - Since then, I have been living in Berlin), or it can be placed in the middle of the sentence (e.g., 'Ich wohne seitdem in Berlin'). Seit cannot be used this way.

2. Tense Selection:

• Main Clause: If a state or habit is still ongoing in the present, the main clause requires the Present Tense (unlike English, which uses the Present Perfect). This applies to both positive states and negated ongoing habits (e.g., 'Ich arbeite seit einem Jahr nicht' - I haven't been working for a year). However, if you are focusing on an event that has failed to occur up to the present moment, or a counted/quantified action within that timeframe, German uses the Perfekt (e.g., 'Ich habe ihn seit einem Jahr nicht gesehen' - I haven't seen him for a year). If a positive state was ongoing at a past reference point, German uses the Präteritum (e.g., 'Als ich ihn traf, wohnte ich seit 2010 in Berlin', or 'Sie arbeitete seit ihrem 19. Lebensjahr in Vollzeit'). The Plusquamperfekt is used when the state had been running right up to that past reference point and then stopped or changed at that very moment (e.g., 'Als er endlich kam, hatte ich seit zwei Stunden gewartet' - When he finally arrived, I had been waiting for two hours). With seit the stretch of time always touches the reference point; it never describes something that had already finished beforehand.
• Subordinate Clause & Mixed Tenses: If the action in the 'since' clause is still ongoing, use the Present Tense. If it refers to a completed event, use the Perfekt or Präteritum. This often creates a sentence with Mixed Tenses (Subordinate = Past, Main = Present). For example: 'Seitdem ich einen Unfall hatte, sitze ich im Rollstuhl' (Since I had an accident, I have been in a wheelchair).

3. Seit vs. Seitdem:

• Usage: As conjunctions, both are grammatically interchangeable, but there is a slight stylistic difference. Seit is much more common in everyday spoken German, whereas seitdem is slightly more emphatic and formal.
• Preposition Use: As mentioned, only seit can be a dative preposition followed by a noun (e.g., 'seit Montag' - since Monday). Seitdem can never be used as a preposition.

4. Common Pitfalls:

• Seitdem vs. seit dem: Do not confuse the one-word conjunction or adverb seitdem with the two-word phrase seit dem, which is the preposition seit followed by the dative article dem (e.g., 'seit dem Tag' - since the day).
• Causal 'Since': German never uses seit to mean 'because'; use da or weil instead.
• Since vs. For: English uses 'since' for a point in time and 'for' for a duration. German uses seit for both! Use it for a point in time (e.g., 'seit 2010' - since 2010) and for an ongoing duration (e.g., 'Ich wohne seit drei Jahren hier' - I have been living here for three years). Do not use für for ongoing durations.
• Seit vs. Seid: Because of terminal devoicing, seit (since) and seid (you all are - from sein) sound identical. Remember: seit is for time, while seid is a verb!