Jumping from one tense to another in a single sentence or from one sentence to another is horrid, but not very common, and a writer will probably notice and fix the problem if she just reads again what she writes.
More common, at least in my copy editing work, is switching tenses between paragraphs. I think this is especially true of research papers, which aren't written as linearly as, say, a short story or a blog post, and which are focused more on the information than on the writing. This is especially a problem when a paper has multiple authors.
Most verb tense problems can be avoided if you do two simple things:
- Think about verb tense before you write. (If you're writing fiction, verb tense is as important as point of view.)
- Read what you wrote before you call it "final."
Of course, doing the latter can help you avoid all sorts of writing problems.
The most common problem I see with verb tenses, though, isn't unexpected shifts between past, present, and future, it's overuse of the progressive form.
In case you've forgotten your ninth-grade English lessons, the progressive form of a verb is created by pairing the conjugated form of to be with the participle (-ing) form of a verb: I was sleeping a minute ago. Now I am screaming at my neighbor's dog. They will be calling the police soon.
Progressive forms have their place and serve a specific purpose. The problem is that people use them too often, making sentences unnecessarily bulky and generally weakening otherwise strong verbs. They're easy to miss when you're writing, too, because they are perfectly grammatical.
But grammatical writing isn't the same as good writing.
Consider these pairs of sentences, the first using the present progressive form and the second using simple present tense:
- When he's drinking, he gets violent.
- When he drinks, he gets violent.
- Giorgio is dead; Michael is running the show now.
- Giorgio is dead; Michael runs the show now.
- Rick is running off at the mouth and deepening our depression.
- Rick runs off at the mouth and deepens our depression.
Here's a verb tense exercise you can try at home to improve your own writing:
Start with something you've already written, just a paragraph or two. Go through and highlight all the forms of to be that you can find. Highlight every is, am, are, was, were, and will be (might as well mark the perfect progressives and grab every has been, and will have been, too). And don't miss the contractions, either.
Now just see how many of them you can get rid of. Replace your "will be going" with "will go," your "was hoping" with "hoped," and your "is flagellating" with "flagellates."
You won't be able to get rid of all of them — and you shouldn't. Not every instance of to be indicates the progressive form. And sometimes the progressive form is exactly what you need. But what I hope you find after this little exercise is more succinct and efficient prose that uses stronger verbs.
And I hope that it's just plain better.
Just remember: There's nothing wrong with the simple past, present, and future tense. Using the simple tense doesn't make you simple any more than the progressive form makes you a progressive or the perfect form makes you perfect. Pay attention to your verbs.