Complete the intergalactic zoologist's field notes about a newly discovered planet by dragging the correct phrases into the blanks.
During the expedition, we encountered five distinct alien species, some of whose defense mechanisms included emitting a scent like burnt toast. We also collected hundreds of peculiar glowing rocks, several of which began to hum softly in the cargo bay.
During the expedition, we encountered five distinct alien species, some of whose defense mechanisms included emitting a scent like burnt toast.
We need a possessive relative pronoun here because the "defense mechanisms" belong to the alien species. "Some of whose" perfectly bridges the gap. "Some of which" would be grammatically broken here, and "some of them" creates a comma splice.
We also collected hundreds of peculiar glowing rocks, several of which began to hum softly in the cargo bay.
When referring to inanimate objects (rocks) in a non-defining relative clause, we use "which." "Whom" is reserved exclusively for people (or highly personified beings), and "many of them" would result in a comma splice because there is no coordinating conjunction.
Relative clause
A relative clause is a dependent clause that modifies a noun, typically introduced by a relative pronoun (who, whom, whose, which, that) or relative adverb (where, when, why). The man who lives in this house has not been seen for days. They split into restrictive (essential to the meaning, no commas) and non-restrictive (extra information, set off by commas).
The split matters because the comma changes the meaning: My brother who lives in Paris (one of several brothers) vs. My brother, who lives in Paris, (my only brother). Getting comma placement right is one of the highest-leverage moves at B2+.
Pronoun
A pronoun is a small, closed class of words that stands in for a noun or noun phrase. The main types: personal (I, you, he, she, it, we, they) plus their object (me, him) and possessive (my, mine) forms; demonstrative (this, that); relative (who, which, that); interrogative (who, what); and reflexive (myself, yourself).
Pronouns are how English avoids endlessly repeating names. The catch: their meaning depends entirely on context, so unclear pronoun reference (Tom told Mike that he was wrong — who's he?) is one of the most common writing problems.
Possessive
The possessive form shows ownership or association in English. With most nouns, you add 's (Sarah's book, the dog's tail); with plural nouns ending in s, you add just an apostrophe (the students' essays). Pronouns have irregular possessives — both possessive determiners (my, your, his, her, our, their) and possessive pronouns (mine, yours, his, hers, ours, theirs).
The most-mixed-up pair: its (possessive of it) vs it's (= it is). Possessive its takes no apostrophe; it's always means it is or it has. Getting this right is one of the highest-leverage punctuation moves in English.
Complex sentence
A complex sentence combines an independent clause with at least one dependent (subordinate) clause: I missed the bus because I overslept. The dependent clause adds extra information — usually about time, reason, condition, or which thing is meant — but can't stand alone. It's introduced by a subordinating conjunction (because, although, if, when, while) or a relative pronoun (who, which, that).
Mastering complex sentences is the move from simple, choppy writing to prose that links ideas. It's also where comma decisions get interesting — placement depends on which clause comes first.
Comma
The comma ( , ) is the most-used punctuation mark in English, separating parts of a sentence where the reader needs a small pause without a full stop. Its main jobs: separating items in a list (apples, pears, and figs), marking off non-essential information (My brother, who lives in Paris, called), and joining clauses with a coordinating conjunction (I went home, and she stayed).
Misuse of the comma — too many, too few, or in the wrong place — is the single most common punctuation issue in English writing. Get it under control and your sentences immediately read more cleanly.
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.
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.