Apply Familiar Endings to فَعَلَ

Three Past-Tense Patterns: فَعَلَ، فَعِلَ، فَعُلَ

The basic verb and its lexical pattern stay the same while the ending identifies the doer:

دَخَلْتُ — I entered
خَرَجُوا — the men went out
فَتَحَتْ — she opened

First recover the basic form: دَخَلَ، خَرَجَ، فَتَحَ. Each is on فَعَلَ.