Die Erdbeeren, die ich gestern gekauft habe, sind süßer als die Himbeeren.

The two die words do different jobs:

• The first die is the definite article for Erdbeeren: die Erdbeeren = the strawberries
• The second die is a relative pronoun: die ich gestern gekauft habe = that I bought yesterday

So the structure is:

• Die Erdbeeren = the strawberries
• die ich gestern gekauft habe = that I bought yesterday

In English, we often use that or which here. In German, the relative pronoun must match the noun it refers to in gender and number, so die is correct for plural Erdbeeren.

Because this is a relative clause, and in German, conjugated verbs usually go to the end of subordinate clauses.

The relative clause is:

• die ich gestern gekauft habe

Its basic pieces are:

• ich = I
• gestern = yesterday
• gekauft habe = bought / have bought

In a main clause, you would say:

• Ich habe gestern Erdbeeren gekauft.

But in the relative clause, the finite verb moves to the end:

• ..., die ich gestern gekauft habe

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

In German perfect tense, you normally have:

• a past participle: gekauft
• an auxiliary verb: habe

In a main clause:

• Ich habe Erdbeeren gekauft.

In a subordinate clause, the auxiliary goes to the end:

• ..., weil ich Erdbeeren gekauft habe
• ..., die ich gestern gekauft habe

So gekauft habe is the normal order at the end of a subordinate clause.

Because the subject is plural: die Erdbeeren.

• singular: die Erdbeere ist ... = the strawberry is ...
• plural: die Erdbeeren sind ... = the strawberries are ...

So sind agrees with the plural noun Erdbeeren.

süßer is the comparative form of süß:

• süß = sweet
• süßer = sweeter
• am süßesten = sweetest

Many one-syllable German adjectives with a, o, u often take an umlaut in the comparative or superlative, if possible. Here, u becomes ü:

• süß → süßer

This is just part of how some common adjectives form comparisons in German.

Because German uses als to make comparisons of inequality:

• größer als = bigger than
• teurer als = more expensive than
• süßer als = sweeter than

So:

• sind süßer als die Himbeeren = are sweeter than the raspberries

This is the standard comparison pattern:

• [thing 1] + ist/sind + comparative + als + [thing 2]

Not exactly. The reason is different in each case.

• die Erdbeeren: plural definite article
• die Himbeeren: also plural definite article

The singular forms are:

• die Erdbeere = the strawberry
• die Himbeere = the raspberry

These nouns are feminine in the singular, but in your sentence they are also plural, and the plural definite article is also die. So die here works for plural nouns.

It is accusative plural.

Why? Because inside the relative clause, it is the direct object of gekauft habe:

• I bought what?
• the strawberries

So the relative pronoun stands for die Erdbeeren and functions as the object in the clause.

You can think of the clause as:

• Ich habe die Erdbeeren gestern gekauft.

When die Erdbeeren becomes a relative pronoun, it becomes:

• ..., die ich gestern gekauft habe

Since plural nominative and plural accusative both use die, the form looks the same, but the function here is accusative.

Because German always sets off relative clauses with commas.

So:

• Die Erdbeeren, die ich gestern gekauft habe, sind süßer als die Himbeeren.

The relative clause is extra information about die Erdbeeren, and German punctuation requires commas around that inserted clause.

This is stricter than English, where punctuation can be less consistent.

Yes, grammatically you could say:

• Die Erdbeeren, welche ich gestern gekauft habe, ...

But in normal everyday German, die is much more common and more natural than welche in relative clauses.

So learners should strongly prefer:

• die
• der
• das

as relative pronouns.

Because gestern is an adverb, and in this relative clause it comes before the final verb cluster.

German word order often places time expressions like gestern somewhere in the middle field of the clause:

• Ich habe gestern Erdbeeren gekauft.
• ..., die ich gestern gekauft habe

You should focus on the bigger rule: in a subordinate clause, the finite verb goes to the end. The adverb gestern naturally appears before that final verb.

Yes. The main clause by itself is:

• Die Erdbeeren sind süßer als die Himbeeren.

That is a complete sentence.

The relative clause:

• die ich gestern gekauft habe

just gives extra information about which strawberries we mean.

So the full sentence is built like this:

• Main clause: Die Erdbeeren sind süßer als die Himbeeren.
• Inserted relative clause after Die Erdbeeren: Die Erdbeeren, die ich gestern gekauft habe, sind süßer als die Himbeeren.

The main statement is in the present tense:

• sind süßer = are sweeter

But the relative clause refers to a completed past action using the perfect tense:

• ich gestern gekauft habe = I bought yesterday / I have bought yesterday

So the sentence combines:

• a present comparison: the strawberries are sweeter
• a past action: these are the strawberries I bought yesterday

That mixture is very normal in German.