The correct form of "to be" is "is" because the subject "He" is third person singular.
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.
Countable and uncountable
Countable vs uncountable: countable nouns can be numbered and pluralised (one book, two books). Uncountable nouns can't (information, not informations). The distinction determines your choice of article, quantifier (much/many, few/little), and whether the noun can be plural.
Countable = takes a/an, has a plural, uses many/few. Uncountable = no a/an, no plural, uses much/little. Some nouns are both depending on meaning (coffee = the substance vs a coffee = a cup).
Diagnostic: can you say one ___, two ___s? Yes → countable. No → uncountable (use a unit phrase: a piece of, a bit of).
Be
Be vs have vs do: all three serve as auxiliaries, but be builds progressives (is running) and passives (was broken); have builds perfects (has gone); do builds negatives and questions (do you, doesn't, did). Only be also works as a main verb (copula: She is a doctor).
The verb be = 8 forms, most irregular English verb. Copula (links subject to complement) + auxiliary for progressive/passive.
Diagnostic: is be followed by an -ing form? → progressive auxiliary. Past participle? → passive auxiliary. Adjective/noun/prepositional phrase? → copula (main 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.
Pronoun
Pronoun vs noun: nouns name explicitly (Sarah, the book). Pronouns substitute and point back (she, it). Pronouns are a closed class (you can't invent new ones easily), while nouns are open (new ones appear constantly). The main complication: pronouns still carry case marking that nouns have lost.
A pronoun replaces a noun or noun phrase. Types: personal, demonstrative, relative, interrogative, reflexive, indefinite.
Diagnostic: every pronoun must have a clear antecedent (the noun it replaces). If the reader can't tell which noun a pronoun refers to → ambiguity error.
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).
Grammatical number
Subject-verb agreement: the verb must match the subject's number, not the nearest noun. The list of items is long (✅) — not are, because the subject is list (singular), not items. This "attraction" error is the most common number mistake.
Grammatical number is the singular/plural system affecting nouns, pronouns, and verbs. Agreement means making them all match.
Diagnostic: find the actual subject (ignore prepositional phrases between subject and verb). Is it singular or plural? Match the verb to that.
A1 | Elementary | Beginners
A1 vs A2: A1 covers isolated survival phrases (Where is…?, I am…, How much?). A2 handles connected sentences about familiar routines and simple past events. If you can manage short fixed phrases but not string together original sentences about your day, you're still A1.
A1 is the entry level of the CEFR: greetings, introductions, numbers, basic present tense, and core function words.
Diagnostic: can you describe yesterday using past tense? No → A1. Yes → you're moving into A2.
Easy
Easy vs Medium vs Hard: Easy = one rule, obvious answer, A1–A2. Medium = one rule but realistic distractors, A2–B1. Hard = interacting rules, edge cases, B2+. Start Easy to check you have the basics before moving up.
The Easy tag filters for single-rule, short-sentence, common-vocabulary challenges designed for beginners or for anyone wanting a confidence check on fundamentals.
Diagnostic: if you get Easy questions wrong, stay here — your foundations need work. If they feel trivial, move to Medium.