I feel one of the skills that shall serve you best in writing is mastering verb tense. I have been asked "what tense do you write in?" and I respond with "the true tense."
Whatever tense you write in, your words must be true in relation to the time in which they happened. A simple example:
She runs down the alley. She had been running for hours. What she didn't know is she would be running until dawn.
Runs is present tense. Had been running is past (perfect progressive) tense, and I could have used ran as well here but liked the sound of had been running too much since it relayed weariness in its verbosity. Would be running is future tense. All three of those statements are true in the present tense.
So I tend to write in present tense by default, and slip into past tense when what I am saying would be related to a past event. Future tense for future events. I keep every statement a truth in regards to the time in which it is said and that keeps me out of trouble.
Now, there are some times you want to flip the past tense switch and stay in that mode. When the story you are relating is narrated and for certain happened in the past in regards to the narrator's experience. Also if you are relating history or past events past tense is a good thing. In this case it is still good to master verb tense because slipping all around with your tense makes you look sloppy.
But even in past tense writing you get dialog lines like, he said, "They are attacking the fort, and they will be breaking into the center keep if we do not act now!" In the dialog you relay a sense of urgency to the words, even though the narrator is saying "said" in past tense, but the dialog is of that one moment and in present, then future tense - because it makes sense and sounds like real dialog that way.
Now, I am not an English teacher, but this way of writing feels natural to me and helps me get through my writing day without having to stop and look up verb tenses every second sentence. I am free to mix tenses in the present tense, but I must do so in a basis of truth in regards to time. If I am writing in past tense, stick to past tense but watch that dialog, and the same rules of truth apply.