A verb is a word used to express an action or a state of being. The tense of a verb indicates the time of the action or state of being that the verb expresses.
Verbs that describe actions or states of being that take place at the same time must be in the same tense.
John whistled while he walks his dog.
(Incorrect. The word while indicates that the two actions expressed by the verbs whistled and walks must take place at the same time. Therefore, they should be expressed in the same tense. However, whistled is in the past tense, while walks is in the present tense.)
John whistles while he walks his dog.
John whistled while he walked his dog.
(Correct. The verbs in these two sentences are consistent in tense; the first sentence locates these actions in the present, while the second locates these actions in the past.)
George wanted to root for the Giants, but his father sees this as an act of betrayal.
(Incorrect. The verbs wanted and sees describe actions that take place at the same time, but these verbs are in different tenses.)
George wanted to root for the Giants, but his father saw this as an act of betrayal.
(Correct. The verbs wanted and saw describe actions that take place at the same time; both are in the same tense, the past tense.)
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