The correct answers are They're planning a surprise party for their mom, There are three cookies left in the jar, and I saw their car parked over there.
"They're" = they are, "their" = possessive (belonging to them), "there" = a place or used with "there is/are." In the incorrect options, "there" should be "their" (possessive), and "Their" should be "They're" (they are).
Adverb
Adverb vs adjective: adjectives describe things; adverbs describe actions, qualities, or degrees. The mix-up usually happens after action verbs — she sings beautiful (wrong) vs she sings beautifully (right).
An adverb modifies a verb, adjective, or another adverb: incredibly fast, she spoke softly, we go often.
Diagnostic: ask what word is this describing? If it's a verb (an action) → adverb. If it's a noun (a thing) → adjective. Exception: linking verbs (be, seem, taste) take adjectives, not adverbs.
Apostrophe
Its vs it's: the single most confused pair in English punctuation. It's = it is (contraction). Its = belonging to it (possessive, no apostrophe — like his or hers).
The apostrophe ( ' ) has exactly two jobs: marking possession (the teacher's desk) and marking contractions (don't, we're). It does not create plurals — two cats, never two cat's.
Diagnostic: try expanding to it is. If the sentence works → write it's. If not → write its. For nouns, ask: does it mean "belongs to"? Yes → add 's. No → just add s for the plural.
Possessive
Noun possessive vs pronoun possessive: nouns ADD an apostrophe for possession (Sarah's, students'). Pronouns NEVER use apostrophes (its, yours, theirs — no apostrophe). This contradiction is why its/it's is the most common error in English writing.
The possessive marks ownership: 's for singular nouns, s' for plural nouns ending in s, and special pronoun forms (my/mine, their/theirs).
Diagnostic: is it a noun? → add 's or s'. Is it a pronoun? → use the built-in possessive form (NO apostrophe). Specifically its (possessive) vs it's (it is).
Pronoun
Pronoun vs noun: nouns name explicitly (Sarah, the book). Pronouns substitute and point back (she, it). Pronouns are a closed class (you can't invent new ones easily), while nouns are open (new ones appear constantly). The main complication: pronouns still carry case marking that nouns have lost.
A pronoun replaces a noun or noun phrase. Types: personal, demonstrative, relative, interrogative, reflexive, indefinite.
Diagnostic: every pronoun must have a clear antecedent (the noun it replaces). If the reader can't tell which noun a pronoun refers to → ambiguity error.
Vocabulary
Vocabulary vs grammar: grammar is the system of rules for combining words. Vocabulary is the stock of words themselves. You can have perfect grammar and still sound limited if your word stock is narrow (good instead of outstanding/remarkable/decent). Most fluency-feel above B1 comes from vocabulary breadth, not grammar complexity.
Vocabulary = word-focused learning: words, collocations, phrasal verbs, idioms, across CEFR A1–C2.
Diagnostic: can you express the idea but it sounds "flat" or overly simple? → vocabulary issue. Can't construct the sentence at all? → grammar issue.
B1 | Intermediate
B1 vs B2: B1 handles standard everyday communication and simple opinions. B2 handles abstract topics, sustained arguments, and nuanced register. If you can chat about your life but struggle to debate an issue or write a formal essay, you're B1.
B1 is the intermediate CEFR level: independent handling of familiar topics, second conditional, basic passive, reported speech, and linking words for cause and contrast.
Diagnostic: can you read a newspaper article on a familiar topic and summarise the argument? Comfortably → B2. Struggle with abstractions → still B1.
Medium
Medium vs Easy: Easy has one obviously correct answer and clearly wrong distractors. Medium has one correct answer but plausible distractors — you need to actually know the rule, not just guess from sound.
The Medium tag filters for A2–B1 challenges with realistic difficulty: one rule per question, plausible alternatives, everyday contexts.
Diagnostic: if you're scoring 90%+ on Easy, move here. If you're below 60% on Medium, go back to Easy for that topic. Target 70–80% accuracy for maximum learning.