German learners often know the word seit very early. You use it when you want to say since or for in German.

But then you see sentences like these:

Seitdem ich Deutsch lerne, verstehe ich mehr.
Seit dem Kurs spreche ich sicherer.

Both look almost the same. Both are correct. But they are not used in the same way.

And this is where many German learners get confused:
When do you write seitdem as one word, and when do you write seit dem as two words?

In this article, you will learn the difference step by step, with simple examples, English translations, and a clear rule at the end.

Recommended study materials on the topic:

Langenscheidt Go Smart - German Verbs
A-Grammar: Practice German grammar German (incl. answers)
German self-study book for A1-B1 (incl. answers)
German Verb Charts: Learning Regular and Irregular Verbs at a glance

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Why “seitdem” and “seit dem” are so confusing

At first, seitdem and seit dem sound almost identical. When you hear them in spoken German, you may not notice any difference.

But in written German, the difference is important.

Look at these two examples:

Seitdem wohne ich in Berlin.
Since then, I have been living in Berlin.

Seit dem Sommer wohne ich in Berlin.
Since the summer, I have been living in Berlin.

The meaning is similar, but the structure is different. In one sentence, seitdem refers to a previous situation. In the other sentence, seit dem is followed by a noun.

Let’s look at both forms more closely.

How to use “seitdem” in German

Seitdem is written as one word.

You can use it when you talk about a change or situation that started in the past and still has an effect now.

“Seitdem” as an adverb

When seitdem is used as an adverb, it means since then. It does not introduce a subordinate clause, so the conjugated verb stays in position 2. You can place seitdem at the beginning of the sentence or later in the sentence (preferably behind the verb), but the verb must still be the second element.

Example 1:
Seitdem spreche ich sicherer Deutsch.
Since then, I speak German more confidently.

Example 2:
Ich spreche seitdem sicherer Deutsch.
I have spoken German more confidently since then.

In the first sentence, seitdem is in position 1, so the verb spreche is in position 2. In the second sentence, ich is in position 1, so the verb spreche is again in position 2.

More examples

Sie wohnt jetzt in Deutschland. Seitdem hört sie jeden Tag Deutsch.
She now lives in Germany. Since then, she listens to German every day.

Wir lernen regelmäßig zusammen. Seitdem machen wir schneller Fortschritte.
We study together regularly. Since then, we make faster progress.

In these examples, seitdem means since then. It connects the second sentence to something that happened before.

“Seitdem” with a subordinate clause

Here, seitdem is a subordinating conjunction, similar to weil, wenn, or obwohl. It introduces a subordinate clause and means since.

Because seitdem introduces a subordinate clause, the conjugated verb goes to the end of this clause. It also means you can start your sentence with “seitdem” but with seitdem as a conjunction, the verb in the seitdem clause always goes to the end, no matter where the clause stands in the sentence. You can also use “seit” instead of seitdem if you use it as a subordinate clause.

Examples

Seitdem / seit ich Deutsch lerne, verstehe ich deutsche Filme besser.
When the subordinate clause comes first, the verb of the main clause comes directly after it.

Ich verstehe deutsche Filme besser, seitdem / seit ich Deutsch lerne.
When the main clause comes first, the word order in the main clause stays normal.

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How to use “seit dem” in German

Now let’s look at seit dem as two words.

Here, seit is a preposition. A preposition usually needs an object, for example a noun or a noun phrase.

In German, seit is always followed by the dative case.

So when you use seit before a noun, the article often changes.

Here, the two words have separate jobs:

seit means since
dem means the in the dative case

So seit dem often means since the.

Examples

Seit dem Kurs spreche ich sicherer.
Since the course, I speak more confidently.

Seit dem Sommer wohne ich in Berlin.
Since the summer, I have been living in Berlin.

Seit dem letzten Jahr lerne ich Deutsch online.
Since last year, I have been learning German online.

Seit dem Gespräch verstehe ich die Situation besser.
Since the conversation, I understand the situation better.

In all these examples, seit dem is followed by a noun or noun phrase:

der Kurs → seit dem Kurs
der Sommer → seit dem Sommer
das Jahr → seit dem Jahr
das Gespräch → seit dem Gespräch

Because seit is a dative preposition, the article changes to dem for masculine and neuter nouns.

But this is important: the two-word form is not always seit dem.

The article depends on the gender and number of the noun.

Conclusion

Seitdem and seit dem are not difficult once you understand the structure. Look at what comes after the word: a full clause, a reference to a previous situation, or a noun.

Use seitdem when it means since then or when it introduces a subordinate clause. Use seit dem when seit is a preposition followed by a noun in the dative case.

If you want to write German more clearly, this small difference is worth learning well.

You can also learn more about the difference between seit and für in German, because both are very important when you talk about time in German.

Bis bald!

Steffie