Learn French: Pendant le trajet, elle est restée seule. - During the journey, she stayed alone.

Because the verb rester uses être as its helper verb in the past tense (passé composé). When a verb uses être, its past participle must agree with the subject. Since the subject elle (she) is feminine, we add an e to resté to make it restée.

Why does this sentence say elle est restée instead of elle a resté?

In French, most verbs use avoir for the past tense, but a specific group of verbs uses être. These are often verbs of motion or a change of state, frequently memorized using the acronym Dr. Mrs. Vandertramp. Rester (to stay) is one of these verbs, so it requires être.

Why is the word for "alone" spelled seule here instead of seul?

Seul is an adjective that must agree with the noun or pronoun it describes. Because it describes elle (a feminine subject), it takes the feminine form by adding an e, becoming seule.

I know pour means "for". Could I use pour instead of pendant to say "during"?