Tense confusions (had, had, had)

Style has nothing to do with content. So it is a choice you make; do you want to write the novel in past tense or present tense. If you mix the two up, the work might be unreadable.

'Had' is what is known as 'past perfect', which the following is an example of:

"He had walked the dog that day."

A normal past tense would be:

"He walked the dog that day."

You can use both in a text; the problem comes when you want to shift between past and present tense.