Choose the wrong tense and your listeners and readers will not understand when an action happened or will happen. Choose the correct tense and everyone is happy.
Use the present simple for things that happen all the time is a good place to start.
The Present Perfect Tense without doubt causes the most confusion. This is looked at over three pages in Present perfect does not talk about the past, Present perfect simple (I have been) vs. past simple (I went), and Present perfect simple (I have written) vs. present perfect continuous (I have been writing).
Going to do vs. will do is another common confusion.
You may need to understand what a Clause is before reading the pages When the main clause is in the past tense and When the main clause is in the future tense.
To finish have a look at the Difference between if I was and if I were.
