The correct answers are The smart fridge that ordered 50 gallons of milk is finally unplugged. and The smart fridge which ordered 50 gallons of milk is finally unplugged.
For defining relative clauses about things, both that and which are perfectly correct.
"Who" is incorrect because a fridge is an object, not a person. The sentence with commas is a non-defining relative clause, and we can never use that after a comma in a non-defining clause.
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.
Punctuation
Punctuation is the set of visual marks — periods, commas, question marks, colons, semicolons, apostrophes, quotation marks, hyphens and dashes — that show readers where sentences begin and end, where pauses go, and how parts of a sentence relate.
Punctuation does two jobs: it follows the rhythm of speech (where you'd pause aloud) and it marks the structure of clauses. Mismatch the two and writing reads either as breathless or as choppy. Mastering the basics is a small investment with huge returns — clear punctuation makes prose look careful and considered.
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.
Sentence
A sentence is the largest grammatical unit in writing — one or more clauses expressing a complete thought, ending with a period, question mark, or exclamation mark. English sentences come in four structural types: simple (one independent clause), compound (two or more independent clauses joined), complex (independent + dependent clause), and compound-complex (multiple independent + dependent clauses).
Mastering sentence types is what lets you vary rhythm in writing. All-simple sentences read as choppy; all-complex sentences read as dense. Mixing them is what makes prose breathe.
B2 | Upper Intermediate
B2 is the upper-intermediate level in the CEFR framework, sitting between B1 and C1. At B2 you can read editorials, follow most TED talks without subtitles, and hold extended conversations on abstract topics — including topics outside your everyday life.
Grammatically, B2 means flexible control of mixed conditionals, passive voice across tenses, reported speech with proper backshifting, and participle clauses. B2 is the standard target for university entrance exams (IELTS 5.5–6.5, TOEFL 87–109) and most skilled-migration thresholds — knowing whether you're there shapes your study plan.
Difficulty: Medium
The Medium difficulty tag marks questions and challenges in the middle of the difficulty range — typically suitable for A2 to B1 learners. Expect a single rule with realistic distractors, longer sentences, and contexts where you have to think before answering rather than reading off the obvious choice.
Filter by Medium when you're past the absolute basics and ready to consolidate. It's the level where most lasting progress happens — easy enough that you can finish without exhausting concentration, hard enough that getting it right means you've actually understood.