Start with the noun you are describing
For a relative pronoun acting as subject, the basic forms are masculine der, feminine die, neuter das and plural die. The relative clause adds information about that noun.
Das ist die Bäckerei, die früh öffnet.
That is the bakery that opens early.
Find the role inside the new clause
If the masculine noun is the direct object inside the relative clause, use accusative den. Feminine die, neuter das and plural die have the same forms in nominative and accusative.
Das ist der Roman, den Nora liest.
That is the novel that Nora is reading.
Do not copy the case from outside
A noun can be an object in the main clause and the subject in the relative clause. The relative pronoun then needs nominative, even when the preceding article is accusative.
Ich nehme den Zug, der um neun abfährt.
I am taking the train that leaves at nine.
The relative pronoun already does a job
Do not add an extra personal pronoun when the relative pronoun is the subject. Keep the finite verb at the end; with a modal, put the action infinitive before it.
Das ist die Ausstellung, die ich besuchen möchte.
That is the exhibition that I would like to visit.
Mark the boundaries with commas
German relative clauses take commas whether they identify the noun or add extra information. If the main sentence continues afterwards, close the relative clause with another comma.
Der Roman, den Nora liest, ist spannend.
The novel that Nora is reading is exciting.
Practise this pattern
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References
Original teaching prose and examples, informed by these grammar references. A2 placement is an editorial curriculum choice, not a CEFR certification.