Verb tense

The verb tenses within a sentence should be compatible. (See Parallelism.)

Margaret wants to go to a movie today, but I wanted to do something outdoors.

The students were whispering to each other when the professor spoke.

As well, you shouldn’t unnecessarily switch verb tenses within the text.

Once upon a time there was a little dog. The dog was black and brown, with some white stripes. The dog likes to walk in the park where he can chase squirrels.