Verb Agreement with Other Verbs
Think of verbs as teammates who can pass the ball to each other so adroitly that the coach would be foolish to have one on the playing field and one on the bench. Verbs are teammates who play so well together that they must play in the same game, at the same time. In the language of grammar, "time" is called "tense." Usually, verbs in sentences agree by being in the same tense.
For example, the following sentences do not make sense since the verbs do not all agree in tense:
The park in Saratoga Springs is overrun with ducks, since the spring water is naturally warm and the residents fed them every day.
Naturalists who work for New York State warn that people should not feed the ducks, since over-feeding causes over-breeding, which in turn fostered disease.
But the sentences do make sense when the verbs agree with themselves in tense.
The park in Saratoga Springs is overrun with ducks, since the spring water is naturally warm and the residents feed them every day.
Naturalists who work for New York State warn that people should not feed the ducks, since over-feeding causes over-breeding, which in turn fosters disease.
Although English has many tenses, the two basic ones are present and past. There's more information, if you think it would be helpful to know more about verb tense.
Exercise 8: Verb Agreement - Tense 
