Grammar (Tenses, Voice, Narration, Determiners, Modals, Prepositions)
Tenses
Present perfect versus simple past is the highest-frequency error. Use present perfect when the time is not stated (“She has submitted the form”); use simple past when the time is named (“She submitted the form on Monday”).
Voice
Passive = correct form of be + past participle, with the object moved to subject position: “The letter was posted by Ravi.” Keep the tense of the active sentence. Only transitive verbs take a passive; “He arrived” has none.
Narration
Shift the tense one step back when the reporting verb is past: present → past, will → would, can → could, present perfect → past perfect. Adjust pronouns, and change time and place words: now → then, today → that day, tomorrow → the next day, here → there, this → that. Questions become statements: “He asked where I lived.”
Determiners
a/an with singular countable nouns (a before consonant sounds, an before vowel sounds); the for something already mentioned or unique. Some in affirmatives, any in negatives and questions. Few/a few with countables, little/a little with uncountables - “a few” and “a little” mean some; “few” and “little” mean almost none.
Modals and prepositions
Must is obligation felt by the speaker; have to is external obligation. Should/ought to give advice. Can is ability, may is permission or possibility. Prepositions of time: at a clock time, on a day or date, in a month, year or long period.
Worked example
Direct: He said, “I will finish the report tomorrow.” Indirect: He said that he would finish the report the next day. Reasoning: past reporting verb → will becomes would; I becomes he; tomorrow becomes the next day.
Traps
Keeping question word order in reported speech (“He asked where did I live”); changing tense after a present-tense reporting verb; retaining “by” when the agent is unknown; confusing few with a few.