Sentences with verbs in the present tense can be negated by adding the words, "does not" immediately before the verb. For example, the sentence, "The boy runs quickly" can be negated by changing it to, "The boy does not run quickly." Note how the form of the verb changes and the letter "s" is dropped from the verb "run" when the negating words "does not" are added. This does not happen when the sentence is negated by the use of the word, "never." The negated sentence becomes: "The boy never runs."
Negation in the past tense is similarly constructed by adding the words, "did not" immediately before the verb. The following sample sentence, "The boy ran quickly" can be negated by changing it to "The boy did not run quickly." Note that the past tense is reflected in the word "did" and the verb "ran" reverts to the form "run." Similar to the sample sentence with the present tense verb, when the sentence is negated by adding the word, "never," the verb "ran" does not change. The resulting negated sentence becomes: "The boy never ran."
Sentences with verbs in the future tense can be negated by adding the words, "not" or "never." In the following example, "The boy will run" may be negated by adding "not" and it becomes "The boy will not run." Note how the verb "run" is unaltered as the future tense is conveyed by the auxiliary verb, "will." Alternatively, the sentence can be negated with the addition of the word, "never." The sentence becomes, "The boy will never run." and again, the form of the verb "run" is not changed.
Conditional statements use the auxiliary verbs, "should" or "would." They can be negated by adding the word "not" immediately after them. For example, the sentence, "The boy should run" becomes, "The boy should not run" when it is negated. Similarly, in the following example, "The dog would run" becomes "The dog would not run" when negated. Similarly, conditional sentences in the past tense are also negated by the addition of the word, "not" but it is added in between the auxiliary and main verb. For example, the sentence, "The dog would have run" becomes "The dog would not have run" when it is negated.