Seitdem ich hier arbeite, spreche ich täglich mehr Deutsch.

Because seitdem is a subordinating conjunction. In German, subordinating conjunctions (like weil, dass, ob, seitdem) send the conjugated verb to the end of the clause.

So:

• Main clause: Ich arbeite hier. → verb in second position
• Subordinate clause: … seitdem ich hier arbeite. → verb (arbeite) goes to the end

The whole sentence starts with the subordinate clause:

Seitdem ich hier arbeite, spreche ich täglich mehr Deutsch.

First part: subordinate clause (verb at the end)
Second part: main clause (verb in second position)

German often uses the present tense with seit / seitdem when an action began in the past and still continues:

• Seitdem ich hier arbeite, spreche ich täglich mehr Deutsch.
  = I started working here in the past and I still work here, and I still speak more German every day.

English prefers the present perfect in this context: “Since I’ve been working here, I’ve spoken more and more German every day.”

You could use the perfect in German, but it changes the nuance a bit:

• Seitdem ich hier arbeite, habe ich jeden Tag mehr Deutsch gesprochen.

This sounds a bit more like a summary of what has happened up to now. The original sentence with spreche focuses more on the current, ongoing situation.

Yes, you can say:

• Seit ich hier arbeite, spreche ich täglich mehr Deutsch.

This is also correct and means the same thing.

Differences:

• seitdem is only a conjunction/adverb and is a bit more explicit, like ever since.
• seit can be:
  • a preposition: seit einem Jahr (for one year / since a year ago)
  • or a conjunction: seit ich hier arbeite (since I have been working here)

In practice, seit and seitdem as conjunctions are very similar here. Seitdem can sound a bit more “complete” or slightly more formal, but both are very common and natural.

German must have a comma between a subordinate clause and a main clause.

• Subordinate clause: Seitdem ich hier arbeite
• Main clause: spreche ich täglich mehr Deutsch

So you must write:

• Seitdem ich hier arbeite, spreche ich täglich mehr Deutsch.

Leaving out the comma is considered a spelling/grammar mistake in standard German.

In the main clause, German has the verb-second rule: the conjugated verb must be the second element.

In Seitdem ich hier arbeite, spreche ich täglich mehr Deutsch:

• First element of the main clause: the entire subordinate clause Seitdem ich hier arbeite counts as position 1.
• Second element: the verb spreche (must come here because of the verb-second rule).
• Then comes the subject ich, and the rest: täglich mehr Deutsch.

So the structure of the main clause is:

1. Seitdem ich hier arbeite (1st position as a block)
2. spreche (verb in 2nd position)
3. ich
4. täglich mehr Deutsch

Ich spreche would put the verb not in second but in third place after the subordinate clause, which breaks the rule.

Yes, that is perfectly correct German:

• Ich spreche täglich mehr Deutsch, seitdem ich hier arbeite.

Now the main clause comes first:

• Main clause: Ich spreche täglich mehr Deutsch
  
  • Ich = element 1
  • spreche = verb in second position
• Subordinate clause: seitdem ich hier arbeite (verb at the end)

The meaning is the same. The difference is mainly emphasis:

• Starting with the seitdem-clause emphasises the time frame or cause.
• Starting with Ich spreche… emphasises what you do (speaking more German).

Yes, täglich mehr Deutsch means that the amount of German you speak is increasing over time:

• literally: “daily more German”
• natural English: “more and more German every day” or “a bit more German each day”

It suggests a gradual increase: each day you speak a little more German than the day before.

If you just want to say you speak a lot of German every day (without the idea of increase), you would say something like:

• Ich spreche jeden Tag viel Deutsch.
  = I speak a lot of German every day.

Deutsch is capitalized because it is a noun meaning the German language.

• Deutsch sprechen = to speak German (the language) → Deutsch is a noun, so it is capitalized.
• deutsche Bücher = German books → deutsche is an adjective, so it is not capitalized.

So:

• Ich spreche Deutsch. (noun, capital D)
• Ich lese deutsche Bücher. (adjective, lowercase d)

In a subordinate clause, the conjugated verb must come at the end. Everything else (subject, objects, adverbs) goes before that final verb.

Correct:

• … seitdem ich hier arbeite.
  Subject (ich) + adverb (hier) + verb at the end (arbeite)

Incorrect or very unnatural:

• ✗ seitdem ich arbeite hier

That word order (… ich arbeite hier) is normal in a main clause:

• Ich arbeite hier.

But in a subordinate clause with seitdem, the verb cannot appear before the end; it has to be final, so hier must come before arbeite.

You could say:

• Seitdem ich hier arbeite, rede ich täglich mehr Deutsch.

This is grammatically correct, but the nuance is slightly different:

• sprechen is the default verb for speaking a language:
  • Ich spreche Deutsch / Englisch / Spanisch.
• reden is more like to talk / to chat, focusing on the act of talking rather than the language as a skill.

In this sentence, sprechen is more natural because you are talking about how much German you use as a language.
With reden, it sounds a bit more like “I talk more in German every day” (more of your talking is done in German). Both can work, but sprechen is the safer, more neutral choice here.