I sometimes have trouble with past and present tense. Yeah, I know. It's one of the easiest things to master. If it happened already, it's in the past. If it's happening now, present tense.
Except it's not that simple. You have to consider what Point of View your story is written in. Are you telling a story that's already happened or happening now? And who's telling it?
First person narrative is when the story is told through a single person. The story is happening or had happened to the narrator. You can usually spot this POV by the use of "I" or "we" as the common pronouns. This is NOT the voice of the author. The story is being told in the character's voice and with his or her personality.
This is the POV I'm currently using for my young adult novel. First person narrative gives the writer a chance to delve into the character's head. Write their innermost reactions and feelings to the world around them. So if a story is being told in the past tense, it's strange to relay reactions that are also in the past tense.
Completely made-up example having nothing to do with my work:
Character 1: "Come on, let's go watch TV."
Narrator: So that's what people did here. They watch TV and eat chips all night. Well, I wasn't into that. "How about a movie?"
The first time you read it, it looks correct, right? It's not.
"That's" a contraction for "that is" which is present tense.
"Watch" is also in present tense. Should be "watched".
"Eat" is in present tense. Should be "ate".
That's the biggest thing messing me up. When I go back to an earlier point in the story to check a fact, I usually find at least one tense error. I've scrubbed the entire piece, make sure to check the work at the end of the day, and still these things slip by. I just don't have an ear for it. I like First Person Narrative and find it a lot easier than Third Person Narrative; if I could get a handle on these reactionary sentences, life would be perfect in the writing world.