The correct answers are NASA has been studying osteoporosis in astronauts for decades to better understand bone density loss. and The WHO recently issued new guidelines for the prevention and treatment of tuberculosis.
There are two distinct rules at play here:
- Abbreviations: Initialisms pronounced letter-by-letter usually take the definite article (the WHO, the FBI, the FDA). Acronyms pronounced as whole words take the zero article (NASA, NATO, UNICEF).
- Diseases: Medical conditions and diseases almost always take the zero article in English (osteoporosis, tuberculosis, cancer), unless you are referring to a specific, restricted instance (e.g., the tuberculosis that spread through the camp).
Article
A/an vs the vs no article: the three-way choice that trips up learners whose first language has no articles (Russian, Japanese, Korean, Mandarin). Each option changes meaning — I saw a dog (any dog) vs I saw the dog (the specific one) vs Dogs are loyal (the species).
Articles are determinatives that mark noun specificity. A/an = indefinite, first mention. The = definite, known referent. Zero article = generic or uncountable.
Diagnostic: ask does the listener already know which one? Yes → the. No, and it's countable singular → a/an. Generic or uncountable → zero article.
Determiner
Determiner vs adjective: both appear before a noun, but determiners specify which/how many while adjectives describe what kind. Determiners come first: the big cat (✅) vs big the cat (❌). You can stack adjectives (big fluffy cat) but generally only one determiner per noun.
A determiner is a function slot before a noun filled by articles, demonstratives, possessives, or quantifiers.
Diagnostic: does the word tell you which one or how many rather than what kind? → determiner. Does it describe a quality? → adjective.
Noun
Noun vs verb: the two core word classes. Nouns name things; verbs describe actions/states. Many English words can be both (run, play, cook, work) — only the sentence slot tells you which role it's playing. The run was exhausting (noun) vs I run every day (verb).
A noun names an entity. It interacts with articles, determiners, forms plurals, and controls verb agreement and pronoun choice.
Diagnostic: can you put the/a before it or pluralise it? → noun. Does it describe an action with tense? → verb. Can it do both? → check the sentence context.
C2 | Proficiency
C2 vs C1: C1 is fluent and effective but occasionally reaches for words or misses cultural nuance. C2 is indistinguishable from a well-read native speaker — idiom, irony, register-switching all land naturally. Most learners never need C2; knowing it exists prevents over-ambition.
C2 is the highest CEFR level: full mastery of idiom, irony, allusion, and rhetorical control across all registers and subjects.
Diagnostic: could your writing pass as a native speaker's in any context — journalism, academia, comedy, legal? Yes → C2. Almost → still C1.
Hard
Hard vs Medium: Medium tests one rule with realistic distractors. Hard tests interacting rules, edge cases, or context-dependent answers where multiple options seem correct until you think deeply. If you're scoring 80%+ on Medium, try Hard to find your real gaps.
The Hard tag filters for B2+ challenges with layered difficulty: rule interactions, subtle distractors, and contexts that demand genuine grammatical reasoning.
Diagnostic: if Hard questions feel impossible, drop to Medium and master the individual rules first. Hard assumes you already know each rule — it tests whether you can apply them together.