You’re not adding HTML code to your file, you’re adding invalid PHP.

To recap, in PHP everything between <?php and ?> is PHP code.

So this is valid PHP:

<?php echo 'hello world';! ?>

This is also valid:

<b><?php echo 'hello world'; ?></b>

This is not valid:

<?php <b> echo 'hello world'; </b> ?>

Why? Because is invalid PHP. It’s a valid HTML tag yes, but you didn’t put it in HTML, you put it in-between the tags so it’s PHP code, and it isn’t valid PHP code.

Sidenotes

• You’re not indenting your code correctly. Any good editor will auto-magically do it for you, PHPStorm has a reformat option, Sublime auto-indents as you type, save yourself time and effort and use an editor that does it for you. Indenting makes your code easier to read, and bugs easier to spot
• Use brackets instead of shorthand, e.g. if( this ) { do that }, it has wider support among editors and checkers. It’s too easy to add stray endwhile; statements ( your original code has 1 while loop, but you close it twice, this makes no sense and only serves to confuse ). If you use { and } a good editor will automatically type the closing bracket for you and indent it accordingly.

So in conclusion, your code should look more like this:

<?php
if ( have_posts() ) {
    echo '<ul>';
    while ( have_posts() ) {
        the_post();
        echo '<li>';
        get_template_part( 'contentArchive', get_post_format() );
        echo '</li>';
    }
    echo '</ul>';
}

Changes:

• Used echo to print out the html
• removed extraneous blank lines and converted to tabs
• fixed indenting
• Used braces instead of PHP shorthand syntax
• removed the trailing PHP closing tag, closing tags are unnecessary at the end of a PHP file if the last thing is PHP code