The correct answers are The dinosaur bones must not be touched by anyone and Flash photography cannot be used in the bat cave.
To form the passive voice with modal verbs, we use modal + be + past participle (e.g., must be touched, cannot be used).
"The historical paintings should not touch by visitors" is incorrect because it is missing the verb "be" and uses the active verb "touch" instead of the past participle "touched."
"Food and drinks must left at the entrance" is incorrect because it is missing the verb "be" (must be left).
Modal verb
Must vs should vs might: the most confused modal trio. Must = strong obligation/near-certainty. Should = advice/expectation. Might = possibility. Getting these wrong changes the force of your statement: You must see a doctor (urgent) vs You should see a doctor (advice) vs You might need a doctor (maybe).
Modal verbs are auxiliaries that encode modality: ability (can), permission (may), necessity (must), advice (should), possibility (might), future (will).
Diagnostic: what meaning are you adding? Obligation → must/have to. Advice → should. Possibility → might/could. Ability → can. Future → will.
Passive voice
Active vs passive: active puts the doer first (The dog bit the man). Passive puts the receiver first (The man was bitten by the dog). Neither is inherently wrong — choice depends on what you want to foreground. Scientific/formal writing uses passive deliberately; vague writing uses it accidentally.
Passive voice = be + past participle. Promotes the object to subject. Good for foregrounding the action/result; bad when it hides who's responsible.
Diagnostic: who's doing the action? If unnamed and that matters → bad passive. If unnamed because it's obvious or irrelevant (The building was constructed in 1920) → good passive.
Negation
Single vs double negatives: standard English uses ONE negative per clause (I don't see anything or I see nothing). Double negatives (I don't see nothing) are grammatical in many languages and some English dialects, but are non-standard in written/formal English. This is the #1 negation trap for speakers of Spanish, Russian, and French.
Negation = not after auxiliary/modal, or do-support. Negative words (never, nobody, nothing) negate alone without adding not.
Diagnostic: count the negatives in the clause. More than one? → double negative. Fix by replacing one with a positive (anything, anyone, ever).
Participle
Present participle vs gerund: both are -ing forms, but a participle acts as an adjective/adverb (the running water, She sat reading), while a gerund acts as a noun (Running is fun). Same form, different grammatical job.
A participle = verb form used as modifier or in compound tenses. Present (-ing): progressive + adjective. Past (-ed/irregular): perfect + passive + adjective.
Diagnostic: is the -ing word describing a noun or modifying a verb? → participle. Is it being a noun (subject, object)? → gerund.
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.