If you are familiar with Bootstrap 3, then you'll notice the CSS form classes are pretty much the same in version 4. The biggest change I see in forms for the new version is that each form group uses a <fieldset> tag instead of <div>. If you are new to Bootstrap forms, a basic form group is made up of a label and an input. It can also include help text, but that is optional. Let's jump right in by creating a generic form that uses a number of core components.
At the very least, a form needs to be made up of one input and one button. Let's start with the basics and create a form following those requirements. Here's the code to get you started:
<form>
<fieldset class="form-group">
<label>Text Label</label>
<input type="text" class="form-control" placeholder="Enter Text">
<small class="text-muted">This is some help text.</small>
</fieldset>
...