I was wondering which of the two sentences are correct:
My story is written in past tense, but character X says this in narration during the morning of the due date, not after, thus giving me the feeling that it should be 'the homework is due today'. However, I've heard of the advise of sticking with a single tense throughout a story, so my mind is telling me to type 'The homework was due today'.
Tenses in direct speech are different from the tense written otherwise. "was" suggests that the character has missed the deadline, while "is" suggests that the deadline has yet to be crossed. Perhaps an opinion, but "was" suggests that the character knows the time of the deadline and it has past, while "is" suggests that either 1) the deadline has yet to be passed, or 2) the exact time of the deadline is unknown.
You write the characters saying what is appropriate for them to say at the moment they are speaking, except when the plot or characterization requires them to speak inappropriately for the circumstances.
That includes the characters using the tense that is correct at the moment they are speaking.
Or in some cases they use an incorrect tense to show that they are too excited to think or speak clearly, or are very young children, or foreigners speaking a language they have not learned perfectly, or ignorant boobs, or something.
Writers usually avoid having characters use incorrect tenses and other features of speech as commonly as usually happens in real life. Thus when they do have characters use incorrect tenses or other features of grammar, it stands out.
I note that in science fiction stories involving time travel it is common for characters to wonder whether the past, present, or future tenses would be more appropriate.
But as a general rule, writers make their characters speak clearly and grammatically so the readers understand what the characters are saying, and that includes making the characters use the correct tense for the moment when the characters are speaking.
