Der Gärtner sagt, wir sollen zuerst die Erde lockern, bevor wir die Tulpen pflanzen.

Here wir sollen means we are supposed to, we should, or we are to.

In this sentence, sollen shows that the gardener is giving an instruction or passing on what should be done:

• Der Gärtner sagt, wir sollen... = The gardener says we should / are supposed to...

It is not exactly the same as wollen (to want) or müssen (to have to).

• wir sollen = someone says this is the right thing to do
• wir müssen = it is necessary / required
• wir wollen = we want to

So sollen often has the idea of advice, instruction, or expectation coming from someone else.

Because the subject is wir.

The verb sollen changes like this in the present tense:

• ich soll
• du sollst
• er/sie/es soll
• wir sollen
• ihr sollt
• sie/Sie sollen

Since the sentence says wir (we), the correct form is sollen.

Because sollen is a modal verb, and modal verbs usually send the main verb to the end of the clause in the infinitive.

So the pattern is:

• wir sollen lockern
• du musst gehen
• ich kann kommen

In this sentence:

• wir = subject
• sollen = finite modal verb
• die Erde = object
• lockern = infinitive at the end

That is normal German word order with modal verbs.

Because bevor introduces a subordinate clause, and in subordinate clauses the finite verb usually goes to the end.

So:

• main clause: wir sollen zuerst die Erde lockern
• subordinate clause: bevor wir die Tulpen pflanzen

Compare:

• Wir pflanzen die Tulpen.
  Normal main clause: the verb is in second position.

• ..., bevor wir die Tulpen pflanzen.
  After bevor, the verb moves to the end.

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

Bevor means before and introduces a clause:

• bevor wir die Tulpen pflanzen = before we plant the tulips

Vorher means beforehand / earlier and is an adverb, not a conjunction.

Compare:

• Wir lockern die Erde, bevor wir die Tulpen pflanzen.
  = We loosen the soil before we plant the tulips.

• Wir lockern die Erde vorher.
  = We loosen the soil beforehand.

So:

• bevor
  • clause
• vorher = adverb

Because German uses commas to separate clauses.

Here, Der Gärtner sagt is one clause, and wir sollen zuerst die Erde lockern is another clause.

So the comma helps mark the boundary:

• Der Gärtner sagt, wir sollen zuerst die Erde lockern...

There is also another comma before bevor, because bevor wir die Tulpen pflanzen is a subordinate clause.

German punctuation is usually stricter than English in this area, especially with subordinate clauses.

German can often omit dass after verbs like sagen, especially in everyday language.

So both of these are possible:

• Der Gärtner sagt, wir sollen zuerst die Erde lockern...
• Der Gärtner sagt, dass wir zuerst die Erde lockern sollen...

Both are grammatical.

The difference is mainly in structure:

1. Without dass

   • wir sollen keeps normal main-clause word order

2. With dass

   • the clause becomes a clear subordinate clause
   • the finite verb goes to the end: ...dass wir zuerst die Erde lockern sollen

The version without dass is very common and natural.

Both are in the accusative case because they are the direct objects of the verbs:

• die Erde lockern = loosen the soil
• die Tulpen pflanzen = plant the tulips

That said, their forms look the same as nominative here:

• die Erde can be nominative or accusative because Erde is feminine
• die Tulpen can be nominative or accusative because Tulpen is plural

So you know they are accusative from their function in the sentence, not from a visible article change.

In this gardening context, lockern means to loosen the soil.

So die Erde lockern means something like:

• loosen the soil
• break up the soil
• make the soil less compact

In gardening English, you might also think of aerating or loosening up the soil, depending on context.

Zuerst means first and tells you the order of actions.

In wir sollen zuerst die Erde lockern, it comes before the object because it modifies the whole action: first we should loosen the soil.

This position is very natural. German adverbs like zuerst, dann, heute, oft can often move around, but the exact position changes the emphasis.

For example:

• Wir sollen zuerst die Erde lockern.
  Neutral and natural.

• Zuerst sollen wir die Erde lockern.
  Stronger emphasis on first.

Both are possible.

Yes, that is also grammatical.

With dass, the clause after sagt becomes a subordinate clause, so sollen moves to the end:

• Der Gärtner sagt, dass wir zuerst die Erde lockern sollen...

Without dass, you get:

• Der Gärtner sagt, wir sollen zuerst die Erde lockern...

Both mean essentially the same thing. The version without dass is often a bit more direct and common in speech.

Because Der Gärtner is the subject of the main clause — the person doing the action of sagt.

So:

• Der Gärtner = the gardener
• sagt = says

If it were the object, you would expect a different form, such as den Gärtner.

Here the sentence starts with the subject, so nominative is required.