"have failed" is the correct option because it is in the present perfect tense, which is used to indicate that an action has been completed in the past but has a connection to the present. "Fail" is in the present simple, which is not appropriate in this context because the action is completed. "Had failed" is in the past perfect, which is used to indicate that an action was completed before another action in the past. This option doesn't match the context of the sentence, as there's no other action in the past that this is being compared to. "Was failing" is in the past continuous, which is used to indicate that an action was in progress at a specific time in the past. But this option doesn't match the context of the sentence, as the speaker is describing the result, not the progress of the action.
Present tense
The present tense in English has four forms: simple present (I work) for habits, general truths, and stative descriptions; present progressive (I am working) for actions happening right now or temporary situations; present perfect (I have worked) for past actions with present relevance; and present perfect progressive (I have been working) for ongoing actions continuing into the present.
The simple/progressive distinction is one of the trickiest jumps for learners — I work in Paris (habitual) and I'm working in Paris (temporary, right now) feel almost identical but signal different things. Pick wrong and your meaning subtly shifts.
Perfect tense
The perfect aspect marks an action as complete relative to a point in time. It's formed with have + past participle: I have eaten (present perfect), She had finished (past perfect), They will have arrived (future perfect). The perfect doesn't just say when — it says the action's completion is relevant to the time of reference.
The trickiest English-specific use is the present perfect: I have lived in Paris connects the past to now (you may still live there), while I lived in Paris doesn't. This connection is one of the biggest jumps for learners whose native language doesn't make the same distinction.
Clause
A clause is a grammatical unit built around a verb — typically a subject plus a predicate (She laughed; The manager approved the budget). Clauses come in two types: independent clauses stand alone as complete sentences; dependent clauses need an independent clause to make sense (Because I overslept — incomplete on its own).
Spotting clause boundaries is the foundation of correct punctuation. Once you can see where one clause ends and another begins, comma rules, run-on sentences, and complex sentence structure stop being mysteries.
Adverb
An adverb modifies a verb, an adjective, or another adverb — adding information about how, when, where, how often, or to what degree something happens: she sings beautifully, unbelievably fast, we go there often. Many adverbs end in -ly, but plenty don't (well, fast, hard, almost).
Adverbs matter because they're how you add nuance without piling on extra clauses. Used well, a single adverb can sharpen a vague sentence (she answered → she answered honestly), but misplace one and the meaning drifts in a way native speakers immediately notice.
C1 | Advanced
C1 is the advanced level in the CEFR framework, sitting between B2 and C2. At C1 you stop translating in your head and start thinking in English — handling specialised articles outside your field, picking up implicit meaning, and writing structured arguments on complex topics.
Grammatically, C1 means natural use of inversion (Rarely have I seen…), mixed and advanced conditionals, subjunctive forms in formal contexts, and cleft sentences for emphasis. Most university programmes for non-native speakers and many professional certifications set C1 as their entry standard.
C2 | Proficiency
C2 is the highest level in the CEFR framework — the proficiency stage, where your English is nearly indistinguishable from a well-educated native speaker's. C2 users handle irony, understatement, and idiomatic range across any register, and they reformulate ideas under pressure without losing fluency.
C2 is less about learning new grammar and more about mastering the flexible, context-sensitive use of everything you already know. Most learners never reach C2 — and most don't need to. Knowing the level helps you set realistic goals: B2 or C1 is plenty for almost any practical purpose.
Difficulty: Hard
The Hard difficulty tag marks questions and challenges aimed at upper-intermediate to advanced learners — typically B2 and above. Expect interacting rules, edge cases, distractors that look right at first glance, and contexts where the surface meaning and the grammatical answer don't match.
Filter by Hard when you're past the basics and want material that genuinely tests your understanding. These questions catch the gaps your textbook didn't — register-sensitive choices, exception cases, mixed conditionals, the difference between would have been and had been.