Learn French: Cependant, le routard a peur d'oublier son carnet. - However, the backpacker is afraid of forgetting his notebook.

Because the person who is afraid (the backpacker) is the same person who might forget. When the subject is the same for both actions, French avoids the subjunctive and uses an infinitive instead.

What would the sentence look like if he was afraid of someone else forgetting the notebook?

If the subjects were different, you would use que followed by the subjunctive. For example, to say 'The backpacker is afraid that I will forget his notebook', you would say Le routard a peur que j'oublie son carnet.

Why is there a d' before oublier?

The base expression is avoir peur de (to be afraid of). When you follow it with an infinitive verb that starts with a vowel, like oublier, the de contracts to d'.

I haven't seen the word routard before. Where does it come from?

Un routard is a common word for a backpacker or budget traveler. It comes from the word route (road), referring to someone who hits the road to explore.

Quick refresher: why do we use son for 'his notebook' here?