Once you are comfortable with parts of speech and tenses, the next layer of grammar is transformations and functional rules: changing Active to Passive, converting Direct to Indirect speech, choosing the right article, framing question tag…
Voice change, narration and conditionals are direct questions in SSC CGL Tier-I/II, Bank PO Mains descriptive sections and most state PSC exams. Articles and question-tag errors are favourites in Spotting Errors.
In Active voice the subject performs the action; in Passive voice the subject receives the action.
Direct speech reports the speaker's exact words within quotation marks; Indirect speech reports them as a paraphrase.
Articles ('a', 'an', 'the') and determiners (some, any, much, many, few, little) specify which noun is being talked about.
A question tag is a short question added at the end of a statement to confirm or check it.