// Create a spacy instance (from spaCy4j)
SpaCy spacy = SpaCy.create(CoreNLPAdapter.forEnglish());

// Instantiate a new Errgent for English
Generator errgent = Errgent.forEnglish(spacy);

// Generate a specific grammatical error in the target doc. Since a
// sentence can contain multiple errors at once, all such possible
// errors are returned.
List<GeneratedError> generatedErrors = errgent.generateErrors(
    "If I were you, I would go home.",
    GrammaticalError.REPLACEMENT_SUBJECT_VERB_AGREEMENT
);

// Print out the results. The markedText() method retrieves the
// erroneous text with the error marked by an asterisk on both sides.
// We can also access the char offsets of the error using charStart
// and charEnd methods of GeneratedError.
for (GeneratedError generatedError : generatedErrors) {
    String text = generatedError.markedText();
    System.out.printf(
        "%s, %s%n",
        text,
        generatedError.error()
    );
}