Learn French: Ce qui est difficile, c'est de serrer ce boulon. - What is difficult is tightening this bolt.

Ce qui translates to "what" when it acts as the subject of the verb that directly follows it. Here, "what" is doing the action of being difficult (qui est difficile). If "what" were the object of the verb—for instance, "what I find difficult"—you would use ce que (Ce que je trouve difficile).

Why do we need the word de before serrer?

When you use the emphasizing structure ce qui... c'est... (what is... is...) and the second part is an infinitive verb, French requires the preposition de. So, "what is difficult is tightening" becomes ce qui est difficile, c'est de serrer.

Could I express this without the Ce qui... c'est... structure?

Yes! You could simply say Il est difficile de serrer ce boulon (It is difficult to tighten this bolt). The structure in this sentence is used specifically to put special emphasis on the tightening action, much like saying "What is difficult is..." in English.

How would I use this lesson's grammar to say "What is difficult is tightening just any bolt"?

You would use the indefinite pronoun n'importe lequel. Because boulon (bolt) is masculine singular, you replace it with lequel. The sentence would become: Ce qui est difficile, c'est de serrer n'importe lequel (What is difficult is tightening just any one [of them]).