Learn French: Il n'a plus la force de marcher. - He no longer has the strength to walk.

Instead of ne ... pas (not), this sentence uses ne ... plus, which means no longer or not anymore. When you use plus for negation, it replaces pas.

Why do we use de before marcher?

In French, when a noun is followed by an infinitive verb to specify its purpose, you usually connect them with de. Here, la force de marcher translates to the strength to walk.

Why does it use la force instead of de force?

Because it refers to a specific ability (the strength to walk), it takes the definite article la. If you wanted to say he has no strength left in general, you would drop the article after the negative and say Il n'a plus de force.

How could I add tellement ... que to this sentence to match the current lesson?

To connect this consequence to a cause using this lesson's grammar, you could add a phrase like Il est tellement fatigué qu'il n'a plus la force de marcher (He is so tired that he no longer has the strength to walk).