Learn Hardcore German: Der Sicherheitsgurt lässt sich leicht einstellen, wenn er zu locker sitzt. - The seat belt can be adjusted easily when it is too loose.

This is the “lassen + sich + Infinitiv” construction (also called Pronominalpassiv). It literally means “allows itself to be adjusted,” but in English you’d say “can be adjusted” or “is easy to adjust.”

Why is there a sich after lässt? Is einstellen reflexive here?

No, the belt isn’t adjusting itself. The sich is part of the pronominal passive form. German uses lassen + sich + Infinitiv to express that something can be done. It’s not a normal reflexive verb.

Why not use the regular passive, like Der Sicherheitsgurt wird leicht eingestellt, …?

You could, but native speakers often prefer lassen + sich for devices or garments because it sounds more natural and indicates “one can do it easily.” The standard passive (wird eingestellt) is more formal and focuses more on the action itself.

Why is sitzt used with locker? Couldn’t I say “wenn er zu locker ist”?

German uses sitzen when talking about how something fits (clothes, belts, hats). So “der Gurt sitzt locker” means “the belt fits loosely.” Saying “ist locker” isn’t wrong, but you’ll sound more idiomatic with sitzen in this context.

What does zu locker mean? Why the zu?

Zu before an adjective means “too.” So “zu locker” = “too loose.” It signals that the belt is looser than it should be.

Why is the wenn-clause at the end? Can I put it at the beginning?

Yes. Subordinate wenn-clauses can go before or after the main clause. At the end is common for new or explanatory information. If you front it, remember the verb-second rule:
• End: Der Gurt lässt sich leicht einstellen, wenn er zu locker sitzt.
• Fronted: Wenn er zu locker sitzt, lässt sich der Gurt leicht einstellen.

Why is the pronoun er used for Gurt?

Gurt is masculine in German (der Gurt), so the corresponding pronoun is er.