The -e ending appears when an adjective is attributive (i.e., placed directly before) a definite noun.
Example: den søte frukten (the sweet fruit).
But in frukten er søt, søt is predicative (after the verb) and follows the predicative-agreement rules (common-singular → no -e).
Invert the subject and verb:
Er frukten søt?
This means “Is the fruit sweet?”
Use the indefinite article en for common gender:
En frukt er søt.
Here en = “a,” frukt remains indefinite, and søt stays in predicative form.
Use the demonstrative denne with the definite noun form:
Denne frukten er søt.
Literally: “This-female common fruit is sweet.”