Der Gärtner sagt, der Dünger sei gut für die Rosen.

Because this sentence uses reported speech. In German, reported speech is often marked with Konjunktiv I.

So:

• Direct statement: Der Dünger ist gut für die Rosen.
• Reported version: Der Gärtner sagt, der Dünger sei gut für die Rosen.

Using sei shows that this is what the gardener says, rather than the speaker directly stating it as a fact.

Sei is the 3rd person singular Konjunktiv I form of sein.

A few forms of sein in Konjunktiv I are:

• ich sei
• du seiest or du seist
• er/sie/es sei
• wir seien
• ihr seiet
• sie/Sie seien

In this sentence, der Dünger is singular, so the form is sei.

Not necessarily.

Its main job here is to mark reported speech, not automatically to say the statement is false. However, it can create a certain distance: the speaker is reporting someone else’s claim and not fully taking responsibility for it.

So the feeling is often:

• ist = more direct, more like the speaker presents it as true
• sei = this is what someone says

Because the sentence has two clauses:

1. Der Gärtner sagt
2. der Dünger sei gut für die Rosen

German requires a comma between the reporting clause and the clause being reported here.

So the comma works like a divider between:

• The gardener says
• the fertilizer is said to be good for the roses

This is a type of indirect speech without dass. In that structure, the clause often keeps verb-second word order:

• der Dünger sei gut für die Rosen

Here, der Dünger is in first position, and sei is in second position.

If you use dass, then the verb goes to the end:

• Der Gärtner sagt, dass der Dünger gut für die Rosen sei.

So both are possible, but the word order changes.

Yes, absolutely.

That version is also correct:

• Der Gärtner sagt, dass der Dünger gut für die Rosen sei.

And in everyday spoken German, many people would also simply say:

• Der Gärtner sagt, dass der Dünger gut für die Rosen ist.

Very roughly:

• ..., der Dünger sei ... = concise, written, formal, classic reported speech
• ..., dass der Dünger gut für die Rosen sei = also formal and clearly reported
• ..., dass der Dünger gut für die Rosen ist = very common in normal speech

Because der Dünger is the subject of the reported clause, so it is in the nominative case.

The reported clause is:

• der Dünger sei gut für die Rosen

Here:

• der Dünger = the thing being described
• sei gut = is good
• für die Rosen = for the roses

Since Dünger is the subject, it stays der Dünger.

Gut für means good for.

The preposition für always takes the accusative case. That is why we get:

• für die Rosen

The noun Rose is feminine:

• singular: die Rose
• plural: die Rosen

So here die Rosen means the roses.

Because in the plural, the definite article is die in both the nominative and the accusative.

So:

• nominative plural: die Rosen
• accusative plural: die Rosen

There is no visible article change here, but the case is still accusative because für requires it.

German often repeats the noun when that makes the sentence clearer.

If you said:

• Der Gärtner sagt, er sei gut für die Rosen

then er could be confusing. Grammatically, it might seem to refer to der Gärtner or der Dünger.

By repeating der Dünger, the sentence is completely clear.

They give useful information about gender, number, and case.

• der Gärtner: masculine singular, nominative
• der Dünger: masculine singular, nominative
• die Rosen: plural; here accusative because of für

Also, the basic dictionary forms are:

• der Gärtner = gardener
• der Dünger = fertilizer
• die Rose = rose

Because all nouns are capitalized in German.

So in this sentence:

• Gärtner is a noun
• Dünger is a noun
• Rosen is a noun

That is why they all begin with capital letters.

A rough English-friendly guide is:

• ä in Gärtner sounds somewhat like the vowel in air, but shorter and tenser
• ü in Dünger has no exact English equivalent; it is like saying ee while rounding your lips

Very rough approximations:

• Gärtner ≈ GERT-ner
• Dünger ≈ DUNG-er, but with a front rounded vowel, not the normal English u

Also remember:

• Gärtner has the stress on the first syllable
• Dünger also has the stress on the first syllable