It is important to use the correct tense and voice in your written work. You will probably need to use different tenses throughout depending upon the context.
Only use first person voice in reflective writing
Academic arguments are not usually presented in the first person (using I), but use more objective language, logic and reasoning to persuade (rather than emotional or personal perspectives).
This may not apply, however, if you are asked to write a reflective report based on your own thoughts and experiences.
Use past tense to speak about your method
If you are writing about an experiment you carried out or a method you used then use the past tense. For example: "Our experiment showed wide variations in results where the variable was altered even slightly."
Use present tense to conclude or discuss established knowledge
If you are writing about established knowledge then use the present tense. For example: "Diabetes is a condition where the amount of glucose in the blood is too high because the body cannot use it properly." (Diabetes UK, 2015)
When you are reporting on the findings or research of others then you should use the present tense. For example, you might write: "Smith's research from 2012 finds that regular exercise may contribute to good cardiovascular health."
When you are writing about your conclusions or what you have found then use the present tense. For example: "In this case there is not a large difference between the two diameter values (from Feret's diameter and calculated equation), which again is probably due to the fact that the average circularity ratio is on the high end of the scale, 0.88, and therefore infers near circular pores."
If you are writing about figures that you have presented in a table or chart then use the present tense. For example: "These figures show that the number of birds visiting the hide increase every year in May..."