To express compulsion / obligation (internal or external) the construction على… أن is used. Sometimes this construction is preceded by the verb يَجِبُ (not inflected), but Arabs often omit it.

After على there may be a suffixed pronoun (attached to على) or idafa or simply a noun - this is how the performer is expressed.

The presence of أن causes the verb to appear in the subjunctive or masdar.

To express compulsion / obligation in the past tense, we just add the verb كان in front of the construction.

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