When people with English as their first language ask me for help with tenses, it usually means they have been corrected at work for switching tenses.
For example, they have switched from past to present tense in one sentence like this:
When I wrote this sentence, I am thinking of something else.
So the writer tries hard to write consistently in the same tense and finds they are in trouble again:
When I wrote this sentence, I was thinking of something else. (So far, so good.) Today I was writing a much better sentence. (Whoops, consistency doesn’t always work.)
The point is that you can switch tenses within sentences or paragraphs or pages—as long as you do it on purpose to show a change in time. There are plenty of good reasons for changing tense quickly. You want to say that because you did something in the past, you have decided you will do something different in the future or that you feel differently about it now. Look at this sentence:
This reports on an executive meeting at which a new policy on writing sentences was decided.
The writer has used present tense to talk about the report she is writing, giving it a lively and current sense. But it would be inaccurate to put the second part of the sentence in the present—the report is happening now, but the meeting happened before.
So change tenses as often as you want to, but check that you really meant what you wrote.
If people with English as a second language ask the same question, it is a much longer answer. Infuriatingly, English has 13 tenses: past, present, future, past perfect, present perfect, future perfect and the continuous form of each of those six.
Be patient with yourself and practise plenty of exercises in tense that you can find in sites like LearnEnglishFeelGood.com