The way we talk is lazy. There's nothing wrong with speaking in easy, simple sentence structures; in fact, it would seem stiff and formal if we tried to speak as though we were in an old play all the time. At the same time, we've got to remember that writing can be so much more interesting, so much more varied, so much more powerful than conventional speech. We've got to remember to strengthen our word choices in our writing, and that starts with strengthening our verbs.
Let me show you an example. Here's a pretty typical sentence, giving us some information:
The boat is in the harbor.
The only verb we get is the verb "to be." It's the kind of verb that just tells us one thing - that a thing, namely the boat, is. It's about as passive a verb as you can get; so the sentence gives us only one piece of information.
Now let's try strengthening that verb a bit:
The boat rocks in the harbor.
Immediately, with one stronger word choice, we get not one piece of information from the sentence but two. We know that the boat is in the harbor, but we also know that it is rocking. It gives place and motion, a much more dynamic view into the scene.
Beginning writers often make this mistake when they're working on description. They put everything in a room on the page in detail, but they don't tell us anything about motion, change, or action. Even a still object isn't just being - it's sitting, or someone has placed it, or it has fallen, or it is covered in dust. There's so much more to tell about any scene than only "to be" can provide. Don't get me wrong - "to be" is a necessary verb. But cutting down on it will instantly strengthen your writing. It'll also do something else.
Look at a photograph or imagine a scene, and try writing a page describing it without using "to be" at all, in any form. You heard me - no "is", no "was", and so on. What you'll discover is that you've have to restructure your sentences. Instead saying, "It is a sunny day", you'll have to say, "The sun blazed high in the sky." You'll have to make the object you're talking about the absolute prime mover of each sentence, doing something dynamically. It will make your scene more dramatic and more active. In the future, feel free to drop "to be" back in your writing - but remember how dynamic things got when you cut back on this necessary but passive verb.