The Sentence Diagram Generator visually represents the grammatical structure of the sentence "A mold had infected a culture dish by mistake."

1. Subject: A mold

   • Modifier: A (article)

2. Predicate: had infected

   • Verb: infected
   • Auxiliary Verb: had

3. Direct Object: a culture dish

   • Modifier: a (article)
   • Noun: culture dish

4. Prepositional Phrase: by mistake

   • Preposition: by
   • Object of the Preposition: mistake

This structure can be visually represented in a diagram format, illustrating the relationships between the subject, predicate, and objects, along with their modifiers and additional phrases.