The correct answers are It is I who am taking full responsibility for the incident. and It was the marketing team that left the milk out overnight.
In formal it-cleft sentences, the verb in the relative clause must agree with the emphasized subject. Because the subject is I, the correct verb is am (not is).
Furthermore, when the emphasized noun phrase functions as the subject of the relative clause (the marketing team left the milk out), you must use that or who, never the object pronoun whom.
Relative clause
If you've ever paused over who vs whom vs that vs which — or wondered whether a comma belongs before who — you've hit the relative-clause puzzle. English makes meaning depend on whether the clause is essential information or just extra; one missing comma can flip the meaning of the whole sentence.
A relative clause is a dependent clause modifying a noun, introduced by a relative pronoun (who, whom, whose, which, that) or relative adverb (where, when, why). Restrictive relatives are essential and unmarked; non-restrictive are extra information and set off with commas.
Complex sentence
If your writing is technically correct but reads like a list of short, disconnected statements — I overslept. I missed the bus. I was late. — you've hit the limit of what simple sentences can do. Complex sentences are how you fuse those into one flowing thought (Because I overslept, I missed the bus and was late). It's the single biggest jump in writing maturity.
A complex sentence combines an independent clause with at least one dependent clause: I missed the bus because I overslept. The dependent clause typically signals time, reason, condition, or describes a noun, and is introduced by subordinating conjunctions (because, although, if, when) or relative pronouns.
Pronoun
If you've ever paused before who vs whom, its vs it's, or me vs I — you've felt how much weight pronouns carry in English. They're tiny words but they're case-sensitive (I vs me), context-dependent, and one of the few places where everyday English still trips careful speakers. Get the common patterns right and you instantly sound more careful.
A pronoun is a closed class of small words that replace nouns or noun phrases. Types: personal (I, you, he…), demonstrative (this, that), relative (who, which), interrogative (who?, what?), reflexive (myself), and indefinite (everyone, nobody).
Subject
If you've ever written The list of items are wrong (should be is wrong) — you've hit the subject-agreement trap. The subject is list, not items, and the verb has to agree with it. Long sentences with prepositional phrases between the subject and verb are where this most often goes wrong, and getting it right is what stops careful readers from flagging your writing.
The subject is the part of a sentence or clause that says who or what the sentence is about. Typically a noun, pronoun, or noun phrase before the verb, controlling the verb's number and person.
Verb
If grammar feels overwhelming, the fix is almost always to focus on verbs first. They carry the action, the time, the mood, and the voice — a single verb form decides whether your sentence reads as past or present, fact or hypothetical, active or passive. Get verbs solid and the rest of grammar suddenly looks much smaller.
A verb expresses action, state, or occurrence — the engine of every English sentence. Most verbs have five forms (base, -s, past tense, past participle, -ing); be has eight; modal verbs have fewer. Verbs carry tense, aspect, mood, and voice.
C1 | Advanced
If you've ever sat through a lecture in English, written a complaint letter, or argued a point in a meeting and come out feeling actually understood — not just tolerated — you've felt what C1 looks like. The level matters because it's where most universities, certifications, and skilled-work environments draw their language line.
C1 is the advanced level in the CEFR framework, demanding fluent and flexible language: inversion for emphasis, mixed and advanced conditionals, formal subjunctive, cleft sentences, and complex nominal phrases — all used appropriately across registers.
Difficulty: Hard
If easy and medium questions are clicking but you still feel exposed in real conversation or formal writing, you've outgrown the basics. Hard material is where the gaps you didn't know you had show up: the distractor that "sounds right", the rule that interacts with another rule, the case where context changes the answer. It's where genuine fluency is built.
The Hard difficulty tag marks upper-intermediate to advanced challenges — typically B2 and above. Interacting rules, edge cases, plausible distractors, and contexts that require genuine understanding rather than surface pattern-matching.