Negative sentences are formed by adding an auxiliary verb and 'not' (or the contraction n't) after the first auxiliary. In simple sentences, you must add a form of 'do' before adding 'not'. For example: 'She walks' becomes 'She does not walk' and 'They were running' becomes 'They were not running.'
Start with simple present tense and add 'not' with do/does, then move to past tense with 'did not', then to sentences with existing auxiliaries like 'was not' or 'have not'. Practice both full forms (do not) and contractions (don't).
Students often put 'not' directly after the main verb without an auxiliary ('She walks not' instead of 'She does not walk'). They may also confuse where to place the 'n't contraction. Some learners think they need to change the main verb form when adding 'not'.
You already know that auxiliary verbs carry meaning about modality, tense, and aspect — things like "can," "will," "have," and "be." Negation in English works by inserting "not" immediately after the first auxiliary in a verb phrase. In "She was running," "was" is the auxiliary, so the negative is "She was not running." In "They have finished," the negative is "They have not finished." The rule is consistent: find the first auxiliary, and place "not" after it.
The complication arises with simple present and simple past tenses, which have no overt auxiliary in their affirmative forms. "She walks" contains only a main verb with no helper. To negate it, English requires do-support: you insert the appropriate form of "do" as a dummy auxiliary, then place "not" after it. "She walks" becomes "She does not walk" — the "does" carries the tense and agreement, and the main verb reverts to its base form. The same logic applies to simple past: "He left" becomes "He did not leave," with "did" absorbing the past tense and "leave" returning to the base form.
This is the heart of why negation trips learners up. The main verb seems to "lose" its tense marking when "do" is inserted — "walks" becomes "walk," "left" becomes "leave." But the tense hasn't disappeared; it has moved from the main verb to the auxiliary. Do-support is the mechanism English uses to give negation a site to attach to when no other auxiliary is present in the affirmative form. Once you understand that auxiliary verbs are the carriers of tense and negation, and that "do" is inserted as a placeholder when none exists, the pattern becomes predictable.
A useful check: identify the first auxiliary and confirm that "not" (or the contraction "n't") follows it — never the main verb directly. "She walks not" is archaic and non-standard; "She not walks" is ungrammatical in modern English. The contracted forms "doesn't" and "didn't" follow the same logic: "n't" attaches to the auxiliary, not to the main verb. Practicing with progressively complex verb phrases — those with modals, perfects, and passives — will make this rule feel automatic.