Okay, this is straight back to junior school English. Suprising how easy it is to slip up with your tenses, though. As their name suggests, past tense views everything as though it has happened, present tense views everything as it happens, future tense views everything as it will or might happen.

Past tense

Nikki had known as she entered the building that something wasn’t right. She’d swallowed a nervous lump in her throat and gently closed the door behind her. She had taken in the inch-thick dust covering every surface and thought the place needed some serious cleaning.

Present tense

Nikki knew as she entered the building that something wasn’t right. She swallowed a nervous lump in her throat and gently closed the door behind her. Man, this place needs some serious cleaning, she thought, as she took in the inch-thick dust covering every surface

Future tense

Nikki would enter the building, despite knowing that something wasn’t right. She imagined herself swallowing a nervous lump in her throat and gently closing the door.

Erm…future tense is not the easiest to write out of context, but some writers manage to use it to fantastic effect. I’m not one of them *smile*. If you can do it more effectively I’d love to post it here with an author’s credit (using the same base para), so send it on through.