Some writers use what is called the historic present tense. Done badly and it looks like you don't know the difference between the past and the present. Done well and it can increase tension. Anthony Horowitz, for example, uses it to great effect.
Compare the following passages.
Alison was scared. She slowly placed one foot in front of the other and edged forward. Her mind was confused. She had found out that her best friend had betrayed her. Her future was at the best bad, at the worst terrible. She wept as she crept along the narrow bridge.
Alison was scared. Slowly placing one foot in front of the other she edges along the narrow bridge. She had found that her best friend betrayed her. Now her prospects were bad, if not terrible. She weeps as she takes her next steps.
Scared, Alison placed one foot in front of the other and edged her way along the narrow bridge. One wrong step would mean death. Finding that her best friend had betrayed her had devastated Alison. Weeping she takes another step. She had to go on.
Alison was terrified. She stood frozen with fear. The platform led to a narrow bridge. She needed to cross it.
One step at a time she makes the journey. Each step is a gamble with death. Each step is accompanied by the racing drumbeat of her heart. Each step is distance between her and her enemies.
As she took the last step the bridge behind her crumbled and fell. She was safe.
Passage one only uses the past tense. Passage two, however, uses two sentences in the present tense. Passage three uses the participles ‘finding’ and ‘weeping’, the conditional modal verb ‘would’ and the present tense verb ‘takes’. Passage four takes a different approach: a whole paragraph is written in the present tense. This is a technique that should only be used very rarely.
Using the present tense can add immediacy and pace. It can make your writing more exciting. It can be used when you are writing in the first person as well as the third.
I couldn’t go on. Exhaustion had chained my body to the spot. But the desire to survive pulls me forward. Crawling on hands and knees I move along the corridor. I can see the door ahead. Pain washes over me. Ten feet. Five feet. Then the door opened and I saw him.
The present tense can also be used in first-person narratives to suggest someone talking to themselves.
I struggled to my feet. I am not going to die on my knees. I am not going to give him the satisfaction. Pain cut like a razor through my thigh muscle, but I would not let him see my weakness.
Some people like to put inverted commas around thoughts like this or use italics when typing.