<!---------使用一般的.row 格線------->
<form action="">
  <div class="row">
  <div class="col">
    <input type="text" class="form-control">
  </div>
  <div class="col">
    <input type="text" class="form-control">
  </div>
</div>  
</form>
<!---------使用一般的.form-row 格線------->
<form>
  <div class="form-row">
  <div class="form-group col-md-6">
    <label for="city" class="col-form-label">city</label>
    <input type="text" class="form-control" id="city">
  </div>
  <div class="form-group col-md-4">
    <label for="country" class="col-form-label">country</label>
    <input type="text" class="form-control" id="country">
  </div>
  <div class="form-group col-md-2">
    <label for="road" class="col-form-label">road</label>
    <input type="text" id="road" class="form-control">
  </div>
</div>  
</form>