使用flex-direction:column使元素垂直而不是水平，并使用display:flex将一些元素 Package 在div中，以便它们在一行中。
在下面的代码片段中，我用“！”突出显示了我对您的代码所做的更改！！”评论。

body {
  background: #ccc;
  font-family: 'Garamond', sans-serif;
  color: #000080;
}

h2 {
  width: 25%;
  text-align: center;
  margin: auto;
  padding: 15px;
}

.mytabs {
  display: flex;
  /*property of flex container to use flexbox*/
  flex-wrap: wrap;
  max-width: 1000px;
  margin: 25px auto;
}

.mytabs input[type="radio"] {
  display: none
}

.mytabs label {
  padding: 25px;
  font-weight: bold;
  background: #e2e2e2;
  font-size: 20px;
}

.mytabs .tab {
  width: 100%;
  background: #fff;
  order: 1;
  display: none;
}

.mytabs input[type='radio']:checked+label+.tab {
  display: block;
}

.mytabs input[type="radio"]:checked+label {
  background: #fff;
}

.form-group {
  display: flex;
  align-items: center;
  justify-content: center;
  flex-direction: column; /* !!! */
  margin: 40px 40px;
  font-size: 20px;
}

.form-group input[type="radio"] {
  display: block;
}

.form-group label {
  padding: 5px;
  margin: 5px;
  font-weight: normal;
  background: #fff;
  font-size: 20px;
}

input[type=text] {
  padding: 6px 8px;
  margin: 8px 0;
  box-sizing: border-box;
  border: 1px solid navy;
  border-radius: 4px;
}

.flex-wrapper { /* !!! */
  display: flex; /* !!! */
} /* !!! */

<br><br>
<h2>Tool</h2>
<div class="mytabs">
  <input type="radio" id="tabfree" name="mytabs" checked="checked">
  <label for="tabfree">Tab1</label>
  <div class="tab">
    <form action="/submitu">
      <div class="form-group">
        <p>Search By :</p>
        <div class=flex-wrapper> <!-- !!! -->
          <input type="radio" id="user" name="srchusr" value="UserID" checked/>
          <label for="user">Option1</label>
          <input type="radio" id="group" name="srchusr" value="Group" />
          <label for="group">Option2</label>
        </div> <!-- !!! -->
        <div class=flex-wrapper> <!-- !!! -->
          <label for="srchtxt">Search Text :</label><br>
          <input type="text" id="srchtxt" name="srchtxt" value="">
          <br>
        </div> <!-- !!! -->
        <input type="submit" value="Submit">
      </div>
    </form>
  </div>
  <input type="radio" id="tabsilver" name="mytabs">
  <label for="tabsilver">Tab2</label>
  <div class="tab">
    <p>Tab2 Data</p>
  </div>
</div>

你可以使用网格。要构建一个页面或一个表单，网格非常有用。

body {
  background: #ccc;
  font-family: 'Garamond', sans-serif;
  color: #000080;
}

h2 {
  width: 25%;
  text-align: center;
  margin: auto;
  padding: 15px;
}

.mytabs {
  display: flex;
  /*property of flex container to use flexbox*/
  flex-wrap: wrap;
  max-width: 1000px;
  margin: 25px auto;
}

.mytabs input[type="radio"] {
  display: none
}

.mytabs label {
  padding: 25px;
  font-weight: bold;
  background: #e2e2e2;
  font-size: 20px;
}

.mytabs .tab {
  width: 100%;
  background: #fff;
  order: 1;
  display: none;
}

.mytabs input[type='radio']:checked+label+.tab {
  display: block;
}

.mytabs input[type="radio"]:checked+label {
  background: #fff;
}

.form-group {
  display: grid;
  grid-template-columns: 1fr 1fr;
  grid-template-rows: repeat(4, 1fr);
  gap: 10px;
  align-items: center;
  justify-content: center;
  margin: 40px 40px;
  font-size: 20px;
}

.form-group>div:nth-child(even) {
  text-align: right;
}

.form-group>div:nth-child(odd) {
  text-align: left;
}

.form-group>div:first-child,
.form-group>div:last-child {
  grid-column: span 2;
  text-align: center;
}

.form-group input[type="radio"] {
  display: inline-block;
}

.form-group label {
  padding: 5px;
  margin: 5px;
  font-weight: normal;
  background: #fff;
  font-size: 20px;
}

input[type=text] {
  padding: 6px 8px;
  margin: 8px 0;
  box-sizing: border-box;
  border: 1px solid navy;
  border-radius: 4px;
}

<br><br>
<h2>Tool</h2>
<div class="mytabs">
  <input type="radio" id="tabfree" name="mytabs" checked="checked">
  <label for="tabfree">Tab1</label>
  <div class="tab">
    <form action="/submitu">
      <div class="form-group">
        <div>Search By :</div>
        <div>
          <input type="radio" id="user" name="srchusr" value="UserID" checked/>
          <label for="user">Option1</label>
        </div>
        <div>
          <input type="radio" id="group" name="srchusr" value="Group" />
          <label for="group">Option2</label>
        </div>
        <div>
          <label for="srchtxt">Search Text :</label>
        </div>
        <div>
          <input type="text" id="srchtxt" name="srchtxt" value="">
        </div>
        <div>
          <input type="submit" value="Submit">
        </div>
      </div>
    </form>
  </div>
  <input type="radio" id="tabsilver" name="mytabs">
  <label for="tabsilver">Tab2</label>
  <div class="tab">
    <p>Tab2 Data</p>
  </div>
</div>