Another convention of Standard Written English is to select a primary tense for an essay and then stick to it unless the logic of what is being said requires a shift. In spoken English, people tend to use tense at least part of the time to indicate intensity rather than time. That is, they speak of an event that occurred yesterday, use the past tense until they get to the really exciting part, and then shift into the present tense:
"The car was going seventy miles an hour and all of a sudden there's a train coming across the road and I slam on the brakes, but they just go through the floor..."
This really should be kept in the past tense in Standard Written English, and would look like this:
"The car was going seventy miles an hour and all of a sudden there was a train coming across the road, and I slammed on the brakes, but they just went through the floor..."
