Feasible does not mean completed
A file that can be opened has not necessarily been opened. Lassen agrees with the subject, while sich stays part of the feasibility construction. Keep this separate from causing or arranging an action, as when a person has a door repaired.
Die Datei lässt sich öffnen.
The file can be opened.
Context distinguishes a requirement from a possibility
A deadline in a formal instruction often supports a requirement reading. Expressions of ease or difficulty often support possibility. With a separable verb, zu appears inside the infinitive; with a simple verb, it remains a separate word.
Der Antrag ist bis Freitag einzureichen.
The application must be submitted by Friday.
Learn an adjective’s established sense
Not every verb produces a natural -bar adjective. Even established examples can have specialised senses. Before a noun, the adjective takes the usual ending; after sein as a predicate, it stays uninflected.
Wir brauchen eine bezahlbare Wohnung.
We need an affordable flat.
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References
Original explanations and examples informed by grammar references. Level placement is an editorial teaching choice.