bem-nesting.html

<nav class="nav">
	<ul class="nav__list">
		<li class="nav-list__item">
			<a class="nav-list-item__link"></a>
		</li>
		<li class="nav-list__item">
			<a class="nav-list-item__link"></a>
		</li>
		<li class="nav-list__item">
			<a class="nav-list-item__link"></a>
			<ul class="nav-list-item__sub-nav-list">
				<li class="nav-list-item-sub-nav-list__item">
					<a class="nav-list-item-sub-nav-list-item__link"></a>
				</li>
			</ul>
		</li>
	</ul>
</nav>

Since nav is the block, I've taken the approach that everything can be just a single element from that. Just becausenav-list__item is underneath the nav__item, doesn't mean it's required to be there. So you don't have to deep nest at all if you don't want to.

<nav class="nav">
    <ul class="nav__list">
        <li class="nav__item">
            <a class="nav__link"></a>
        </li>
        <li class="nav__item">
            <a class="nav__link"></a>
        </li>
        <li class="nav__item">
            <a class="nav__link"></a>
            <ul class="nav__sub-nav-list">
                <li class="nav__item">
                    <a class="nav__link"></a>
                </li>
            </ul>
        </li>
    </ul>
</nav>

If your sub-nav-list needs special styling, then you can just create the modifier for it and then style your nav__item and nav__link according to the modifier.

<ul class="nav__list nav__list--sub-nav">
    <li class="nav__item">
        <a class="nav__link"></a>
    </li>
</ul>