Sobald wir alles gepflanzt haben, setzen wir uns auf die Bank und trinken Tee.

Because sobald is a subordinating conjunction. In German, subordinating conjunctions push the finite verb to the end of the subordinate clause.

So:

• Wir haben alles gepflanzt. = main clause
• Sobald wir alles gepflanzt haben, ... = subordinate clause

In the subordinate clause, the verb group goes to the end, which is why you get gepflanzt haben.

In a normal main clause, the perfect tense is:

• Wir haben alles gepflanzt.

But in a subordinate clause, German moves the whole verbal part to the end, and the usual order is:

• past participle + auxiliary
• so: gepflanzt haben

That is standard German word order in subordinate clauses.

German often uses the present tense and the perfect tense to talk about future situations when one action will be completed before another.

Here, the idea is:

1. first we finish planting everything
2. then we sit down and drink tea

So Sobald wir alles gepflanzt haben means something like As soon as we have planted everything.

This is very natural in German. You do not usually need a future tense like werden pflanzen here.

Because the sentence begins with a subordinate clause:

• Sobald wir alles gepflanzt haben, ...

That entire clause takes the first position in the sentence. In German main clauses, the conjugated verb must come in second position, so after the opening subordinate clause, the next thing must be the verb:

• ..., setzen wir uns auf die Bank ...

If the sentence started directly with wir, then you would say:

• Wir setzen uns auf die Bank und trinken Tee.

Because sich setzen is a reflexive verb. It means to sit down or to seat oneself.

For wir, the reflexive pronoun is uns:

• ich setze mich
• du setzt dich
• wir setzen uns

So wir setzen uns = we sit down

This is a very common and important distinction:

• sich setzen = to sit down
  (movement, change of position)
• sitzen = to be sitting / to sit

So in this sentence:

• setzen wir uns auf die Bank = we sit down on the bench

If you wanted to describe the result afterward, you would say:

• Wir sitzen auf der Bank. = We are sitting on the bench.

Because auf is a two-way preposition. It can take:

• accusative for movement/change of position
• dative for location

Here there is movement: we are sitting down onto the bench.

So:

• auf die Bank = onto the bench / to the bench position

But if you are already there, you would say:

• auf der Bank = on the bench

Compare:

• Wir setzen uns auf die Bank. = We sit down on the bench.
• Wir sitzen auf der Bank. = We are sitting on the bench.

Because the same subject continues for both verbs:

• setzen wir uns auf die Bank und trinken Tee

German, like English, often leaves out the repeated subject when two verbs are connected and clearly have the same subject.

You could repeat it for emphasis, but normally you would not:

• natural: ..., setzen wir uns auf die Bank und trinken Tee
• also possible but less smooth: ..., setzen wir uns auf die Bank und wir trinken Tee

Because Tee is being used as a general mass noun here, like tea in English.

• Tee trinken = to drink tea

No article is needed when you mean the drink in a general sense.

If you wanted a different meaning, you might use an article:

• einen Tee trinken = have a tea / drink a cup of tea
• den Tee trinken = drink the specific tea already mentioned

So trinken Tee is perfectly normal here.

Yes. In German, a subordinate clause is normally separated from the main clause by a comma.

So this is correct:

• Sobald wir alles gepflanzt haben, setzen wir uns auf die Bank und trinken Tee.

The comma marks the end of the sobald clause and the start of the main clause.

You could, but the meaning changes slightly.

• sobald = as soon as
• wenn = when (sometimes if in other contexts)

So:

• Sobald wir alles gepflanzt haben, ... emphasizes that the next action happens immediately after the planting is finished.
• Wenn wir alles gepflanzt haben, ... is possible, but it sounds a little less precise and less immediate.

In this sentence, sobald is the better choice if you want the sense of as soon as.

Alles means everything, and here it is the direct object of gepflanzt haben.

So the basic idea is:

• wir haben alles gepflanzt = we have planted everything

That means alles is in the accusative, although with alles the form does not change. It refers to all the things that need planting.