The correct answers are We have been watching the suspect's house since midnight and We have identified three new accomplices so far.
Present Perfect Continuous is used for ongoing actions that started in the past and are still happening (like watching the house).
Present Perfect Simple is used when we focus on the result or a specific quantity of a completed action (like identifying three accomplices).
"Know" is a stative verb, so it cannot be used in the continuous form (it should be have known).
Verb
Verb vs noun vs adjective: nouns name things. Adjectives describe. Verbs express what happens or what IS. The test: can it take tense (walked, will walk)? Can it take -ing? Can it follow to as an infinitive (to walk)? Yes to any → verb. English often converts freely between classes (run = noun or verb), so context decides.
A verb = action/state/occurrence word. 5 forms (base, -s, past, past participle, -ing). Carries tense, aspect, mood, voice. The one required element in every sentence.
Diagnostic: does it change for tense (walk → walked)? Can you put to before it (to walk)? Does it take -ing (walking)? → verb.
Present tense
Simple present vs present progressive: simple present = habits, routines, permanent facts (I work here). Present progressive = right now, temporary, changing (I'm working from home today). The most common confusion: using progressive for habits (I'm working here ❌ for permanent job) or simple for right-now (I work now ❌ for current activity).
The present tense has four forms: simple, progressive, perfect, perfect progressive — each relating the action to "now" differently.
Diagnostic: is it a habit/permanent fact? → simple. Happening right now? → progressive. Started in past but still relevant? → perfect. Ongoing duration up to now? → perfect progressive.
Perfect tense
Present perfect vs simple past: I lost my keys (past: specific time, done). I have lost my keys (perfect: result matters NOW — I still don't have them). The perfect always connects past action to present relevance. If the time is specified (yesterday, in 2010) → simple past. If the result matters now → present perfect.
The perfect aspect = have + past participle. Marks completion relative to a time point. Three forms: present/past/future perfect.
Diagnostic: does the sentence mention a specific finished time (yesterday, last year, in 1999)? → simple past. Is it about the result/relevance NOW? → present perfect.
Progressive tense
Progressive vs simple: I work in London (permanent job) vs I am working in London (temporary assignment). Simple = fact/habit/permanent. Progressive = ongoing/temporary/in-progress. Same verb, different aspect, different meaning. The choice isn't about grammar preference — it changes what you're communicating.
The progressive = be + -ing. Marks ongoing/temporary actions. Stative verbs resist it.
Diagnostic: is the action happening RIGHT NOW and likely to stop? → progressive. Is it a general truth, habit, or scheduled event? → simple. Is the verb stative (know, own, believe)? → simple (even if happening now).
Sentence
Sentence vs clause vs phrase: a phrase has no subject-verb pair. A clause has subject + verb. A sentence is one or more clauses packaged with end punctuation as a complete thought. These three levels — phrase ⊂ clause ⊂ sentence — are the structural hierarchy of English.
A sentence is the largest grammatical unit: one+ clauses ending with a period, question mark, or exclamation mark. Four structural types: simple, compound, complex, compound-complex.
Diagnostic: does it have at least one independent clause AND end punctuation? Yes → sentence. Missing independent clause? → fragment. Missing end punctuation? → run-on.
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.