The correct answers are might have made, could explode, and had to evacuate.
Here is how the engineer's original panicked words shift in the captain's past-tense report:
- "I may have made a miscalculation." (Past possibility). The modal may backshifts to might, keeping the perfect infinitive (have made).
- "The reactor could explode!" (Present/future possibility). The modal could does not backshift; it remains could. Selecting "could have exploded" would incorrectly imply the danger had already passed and the explosion didn't happen.
- "You must evacuate!" (Present/future obligation). When must is used for obligation or commands, it backshifts to had to. "Must have evacuated" would wrongly change the meaning to a past deduction (e.g., "I deduce that you evacuated").
Indirect speech
- Direct: "I am tired." → Indirect: She said she was tired. (present → past)
- Direct: "I will come." → Indirect: He said he would come. (will → would)
- Direct: "I have finished." → Indirect: She said she had finished. (present perfect → past perfect)
- today → that day; here → there; tomorrow → the next day
Indirect speech reports someone's words without quotation marks. The mechanism: backshift tenses one step into the past, shift pronouns, and adjust time/place expressions.
Rule: if the reporting verb is past (said, told, asked), shift the reported tense back one step. If the reporting verb is present (says), no shift needed.
Modal verb
- ✅ She can swim. — ❌ She can to swim. (modal + bare infinitive, no to)
- ✅ You must leave now. — strong obligation
- ✅ It might rain. — possibility (~50%)
- ✅ He should apologise. — advice/recommendation
Modal verbs (can, could, may, might, must, shall, should, will, would) are auxiliaries expressing ability, permission, possibility, obligation, or speculation. Always + bare infinitive. Never inflected (she can, not she cans).
Rule: modals never take to after them, never add -s for third person, and can't combine directly (must can ❌ — use must be able to).
Verb
- walk → walk / walks / walked / walked / walking (5 forms, regular)
- go → go / goes / went / gone / going (5 forms, irregular)
- be → am/is/are/was/were/be/being/been (8 forms)
- can → can / could (modal: only 2 forms, no -s, no -ing)
A verb is the one word class every English sentence requires. Carries tense (when), aspect (duration), mood (attitude), and voice (active/passive). Regular verbs add -ed; ~200 irregular verbs have unpredictable past forms.
Key insight: fix your verbs and most grammar problems disappear. Wrong tense, wrong agreement, wrong form — verb errors account for the majority of grammatical mistakes.
Past tense
- I walked home. — simple past (completed action)
- I was walking when it rained. — past progressive (in progress)
- I had already left when she arrived. — past perfect (earlier past)
- I had been waiting for an hour. — past perfect progressive (duration up to a past point)
Four past tense forms: simple past (done), past progressive (was happening), past perfect (had already happened), past perfect progressive (had been happening). Each encodes different timing relative to other past events.
Pattern: simple past = the story's main timeline. Past progressive = background action. Past perfect = flashback to something even earlier.
Infinitive
- ✅ I want to go. — to-infinitive after want
- ✅ She can swim. — bare infinitive after modal
- ✅ Let me help. — bare infinitive after let
- ❌ I enjoy to read. — wrong (enjoy takes gerund, not infinitive)
The infinitive has two forms: to-infinitive (to go) after verbs like want, decide, plan, hope; bare infinitive (go) after modals and causatives (let, make, help).
Rule: after want, need, decide, plan, hope, expect, agree, refuse → to-infinitive. After can, will, must, let, make → bare infinitive. After enjoy, avoid, finish → gerund, NOT infinitive.
C1 | Advanced
- ✅ Not only did she finish early, but she also helped others. — inversion for emphasis
- ✅ It is the process that matters, not the result. — cleft sentence
- ✅ I insist that he be present. — formal subjunctive
- ✅ Were I to disagree, I would say so. — inverted conditional
These are C1 structures — the CEFR advanced level. At C1 you control inversion, cleft sentences, subjunctive forms, and register-switching fluently across formal and informal contexts.
Marker: if you can restructure a sentence for rhetorical effect without hesitation, you're C1.
Hard
- Had she not intervened, the situation would have escalated. — inverted conditional
- All distractors are grammatically plausible in other contexts
- Multiple rules interact (e.g., tense + aspect + modality)
- Context determines the answer — no single "rule" is enough
Hard marks upper-intermediate to advanced challenges: B2+, interacting rules, edge cases, plausible distractors, and contexts where pattern-matching fails.
Use "Hard" when Easy/Medium feel trivial and you want to test whether you actually understand a rule versus just recognising surface patterns.