In "An Occurrence at Owl Creek Bridge", what is the effect of the shift into the present tense in paragraph 36?
I think you may have your question confused somewhat - this short story actually contains no shift in tenses, as your question seems to suggest. Instead it includes very interestingly a shift in the point of view of the narration, which I am guessing your question should actually refer to, and which I will focus on. Consider this following example that contains the shift in point of view that is characteristic of this somewhat confused tale:
The hunted man saw all this over his shoulder; he was now swimming vigorously with the current. His brain was as energetic as his arms and legs; he thought with the rapidity of lightning.
"The officer," he reasoned, "will not make that martinet's error a second time. It is as easy to dodge a volley as a single shot. He has probably already given the command to fire at will. God help me, I cannot dodge them all!"
What is interesting about this extract is the coherent thought and clarity displayed in the thinking of the main character - we shift from the omniscient narrator to being able to see directly into his mind and to be able to trace his thinking. This clearly creates the effect of demonstrating how distorted Farquhar's perceptions are - the author is wanting to hint at the unreality of these events as we press on as readers to the shocking finale of this story.
Hope this helps you to analyse other parts of this excellent short story.