51IMsRplD4L._SY300_As you’ve probably already gathered, the English Test will never explicitly ask you to name a grammatical error, reads SparkNotes.  “But in order to identify and fix errors, you should know what they are. While you’ll often be able to rely on your ear to detect errors, many of the questions will ask you to fix phrases that are fine for spoken English but not for formal written English.”
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The ACT test description states:
Spelling, vocabulary, and rote recall of rules of grammar aren’t tested. See sample questions or read tips and strategies.The test consists of five prose passages, each one accompanied by multiple-choice test questions. Different passage types are included to provide variety.
Some questions refer to underlined portions of the passage and offer several alternatives to the underlined portion. You must decide which choice is most appropriate in the context of the passage.
Some questions ask about an underlined portion, a section of the passage, or the passage as a whole. You must decide which choice best answers the question posed.
Many questions include “NO CHANGE” to the underlined portion or the passage as one of the choices.
The questions are numbered consecutively. Each question number corresponds to an underlined portion in the passage or to a box located in the passage.
Several  grammar issues, appear on the ACT English Test, including:
  1. Subject-Verb Agreement
  2. Pronoun-Antecedent Agreement
  3. Pronoun Cases
  4. Verb Tenses
  5. Adverbs and Adjectives
  6. Idioms
  7. Comparative and Superlative Modifiers
RECOMMENDED READING: Grammar Workbook for the SAT, ACT, and More, by George Ehrenhaft Ed.D.
A solid command of English grammar is a prerequisite for success when students take the SAT, ACT, and other college entrance tests. This workbook presents a detailed grammar review with dozens of practice quizzes and exercises to sharpen students’ skills. It begins with an explanation of grammatical terms and their functions. It then describes the 24 most common mistakes made by students and shows how to avoid them. Prominent among them are lack of agreement between subject and verb, misuse of pronouns, faulty parallel structure, and faulty punctuation. A final chapter focuses on advice and instruction for writing error-free SAT and ACT essays. This workbook is filled with sample questions exactly like those found on both the SAT and ACT, and comes with fully explained answers.
FURTHER RESOURCES:
Pronoun Case Quiz (Guide to Cases of Nouns and Pronouns with two quizzes) http://grammar.ccc.commnet.edu/grammar/cases.htm
Verb Tense Quiz: http://grammar.ccc.commnet.edu/grammar/quizzes/tenses/tenses_frame.html