Der Vermieter sagt, dass der Vertrag nur befristet ist, aber vielleicht später unbefristet werden kann.

In German, a subordinate clause introduced by dass must be separated from the main clause with a comma.

So:

• Der Vermieter sagt = main clause
• dass der Vertrag nur befristet ist ... = subordinate clause

That is why you write:

Der Vermieter sagt, dass ...

This is one of the most important comma rules in German.

Because dass introduces a subordinate clause, and in German subordinate clauses the conjugated verb usually goes to the end.

Compare:

• Main clause: Der Vertrag ist nur befristet.
• Subordinate clause: ..., dass der Vertrag nur befristet ist.

So the verb ist moves from second position to the end of the clause.

No, because aber is still inside the dass-clause.

The whole subordinate clause is:

dass der Vertrag nur befristet ist, aber vielleicht später unbefristet werden kann

Even after aber, you are still inside a subordinate clause, so the verb still stays at the end.

That is why you get:

• ... befristet ist
• ... unbefristet werden kann

not main-clause word order.

Because in a subordinate clause with a modal verb, both verbs go to the end, and the infinitive usually comes before the conjugated modal verb.

Here:

• kann = conjugated modal verb
• werden = infinitive

So the normal order is:

... unbefristet werden kann

Compare:

• Main clause: Der Vertrag kann später unbefristet werden.
• Subordinate clause: ..., dass der Vertrag später unbefristet werden kann.

These are very common words in contract and job contexts.

• befristet = fixed-term, limited in duration
• unbefristet = open-ended, permanent, not limited by an end date

So:

• ein befristeter Vertrag = a fixed-term contract
• ein unbefristeter Vertrag = a permanent/open-ended contract

The prefix un- often makes a word negative or opposite in meaning, similar to un- in English words like unclear.

Because they are not being used directly before a noun here. They are used predicatively, after verbs like sein or werden.

Compare:

• Der Vertrag ist befristet.
  No adjective ending, because befristet comes after ist.

• der befristete Vertrag
  Here you do need an adjective ending, because the adjective comes before the noun.

The same applies to unbefristet:

• Der Vertrag kann unbefristet werden.
• ein unbefristeter Vertrag

nur means only or just.

In this sentence, it emphasizes that the contract is only fixed-term at the moment, not already permanent.

So the idea is:

• right now: fixed-term
• maybe later: permanent/open-ended

Without nur, the sentence would still make sense, but nur adds the feeling of limitation.

Because der Vertrag is the subject of the subordinate clause.

In:

dass der Vertrag nur befristet ist

the thing that is fixed-term is der Vertrag, so it is the subject.

Likewise in:

... aber vielleicht später unbefristet werden kann

the thing that can become open-ended is still der Vertrag.

So der Vertrag stays nominative.

A useful way to see it:

• Who/what is fixed-term? Der Vertrag
• Who/what can become open-ended? Der Vertrag

That makes it the subject.

Here werden is best understood as become.

So:

unbefristet werden = to become open-ended/permanent

That said, in real-life usage this can sound close to a passive idea in English, because a contract does not literally decide by itself to become permanent. In practice, the meaning is that it can later be converted into a permanent contract.

So a natural English translation could be either:

• can later become permanent
• can later be made permanent
• can later be converted into an open-ended contract

But grammatically, in the German sentence, werden is part of unbefristet werden.

They do different jobs:

• vielleicht = maybe, perhaps
• später = later

So:

• vielleicht tells you it is uncertain
• später tells you the time is in the future

Together, vielleicht später means something like maybe later.

Yes. You could say:

Der Vermieter sagt, dass der Vertrag nur befristet ist, aber dass er vielleicht später unbefristet werden kann.

That is also grammatical.

However, German often leaves out the second dass if the structure is already clear. The original sentence sounds natural and less heavy.

If you add the second dass, many speakers would also add er to repeat the subject more clearly.

Because aber is connecting two clauses here:

• der Vertrag nur befristet ist
• (der Vertrag) vielleicht später unbefristet werden kann

German normally uses a comma before aber when it links clauses like this.

So the punctuation helps show the two parallel ideas:

• right now it is fixed-term
• but maybe later it can become open-ended

Yes. The sentence is in the present tense because it reports what the landlord says now.

Possible variations:

• Der Vermieter sagte, dass ... = The landlord said that ...
• Der Vermieter hat gesagt, dass ... = The landlord has said that ...

The rest of the sentence would stay basically the same, with the dass-clause still using verb-final word order.