First check where the adjective stands
Compare Der Tisch ist rund with der runde Tisch. In the first sentence, the adjective describes the subject after ist. In the second phrase, it stands before the noun and needs an ending.
Der runde Tisch steht am Fenster.
The round table stands by the window.
Nominative singular: -e
The subject patterns are der runde Tisch, die runde Lampe and das runde Fenster. The gender changes the article, but the adjective ending is -e in all three.
Die runde Lampe ist hell.
The round lamp is bright.
Masculine accusative: -en
When a masculine phrase is the accusative object, use den runden Tisch. Feminine and neuter keep -e: die runde Lampe, das runde Fenster. Establish the case from the sentence before selecting the ending.
Ich sehe den runden Tisch.
I can see the round table.
Plural: -en in both cases
After plural die, the adjective takes -en in nominative and accusative: die runden Tische. An adjective ending alone therefore does not identify a unique case.
Die runden Tische sind frei.
The round tables are available.
Dieser and welcher follow the same pattern
Use dieser runde Tisch, diesen runden Tisch and welche runden Tische. Do not extend this to every determiner: compare ein runder Tisch, which belongs to a different pattern.
Welchen runden Tisch meinst du?
Which round table do you mean?
Practise this pattern
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Original teaching prose and examples, informed by these grammar references. A2 placement is an editorial curriculum choice, not a CEFR certification.