Der Garten ist so matschig, dass wir heute nicht dort sitzen.

It creates a result clause.

• so + adjective, dass + clause means so ... that ...
• It shows a consequence of what came before.

So here, so matschig, dass ... means the garden is muddy to such a degree that a result follows: we cannot / do not sit there today.

Because dass is a conjunction meaning that.

Compare:

• dass = that as a linker between clauses
• das = the or that/which as an article or pronoun

In this sentence, dass introduces the second clause, so dass is the correct form.

In German, a clause introduced by dass is a subordinate clause, and subordinate clauses are normally separated by a comma.

So:

• Der Garten ist so matschig,
• dass wir heute nicht dort sitzen.

This comma is required in standard German.

Because dass sends the verb to the end of its clause.

German word order changes in subordinate clauses:

• Main clause: the conjugated verb is usually in second position
  • Der Garten ist ...
• dass clause: the conjugated verb goes to the end
  • ..., dass wir heute nicht dort sitzen

That is one of the most important German word-order rules.

Because it comes after ist and works as a predicate adjective.

After verbs like:

• sein
• werden
• bleiben

German adjectives usually do not take endings.

Compare:

• Der Garten ist matschig.
• der matschige Garten

In the second example, the adjective is directly before the noun, so it needs an ending.

matschig means something like muddy, slushy, soggy, or wet and soft in an unpleasant way.

For a garden, it suggests the ground is so wet and muddy that sitting there would be uncomfortable or impractical. It is stronger and messier than just wet.

Yes, the middle part of a German clause is often somewhat flexible.

This version is natural:

• wir heute nicht dort sitzen

Why it works well:

• heute often comes early
• nicht dort keeps the negation close to dort
• the verb still stays at the end because of dass

You may also hear other versions, such as ..., dass wir dort heute nicht sitzen, but the given order sounds normal and clear.

Because nicht often comes before the part being negated.

Here, nicht dort means not there. The idea is that sitting in that place is what is not possible or not happening.

German negation is very sensitive to emphasis and focus, so position matters more than in English.

Yes, both are possible.

• dort = there, slightly more explicit or a bit more formal/written
• da = there, often more conversational
• im Garten = in the garden, if you want to repeat the noun instead of referring back to it

So these are all possible, depending on style:

• ..., dass wir heute nicht dort sitzen
• ..., dass wir heute nicht da sitzen
• ..., dass wir heute nicht im Garten sitzen

The original sentence uses dort to avoid repeating Garten.

Because sitzen and setzen are different verbs.

• sitzen = to be sitting
• setzen = to set / to seat / to sit down something or someone, or oneself in some contexts

Here the sentence is about the state or activity of being seated, so sitzen is correct.

Compare:

• Wir sitzen im Garten. = We are sitting in the garden.
• Wir setzen uns in den Garten. = We sit down in the garden.

That is very normal in German.

German often uses the present tense for the present and the near future, especially when there is a time word like heute, morgen, or später.

So wir sitzen heute nicht dort can naturally mean:

• we are not sitting there today
• we won’t sit there today

The time expression heute makes the intended time clear.