Allen and Greenough's New Latin Grammar, section 194

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There are two Periphrastic Conjugations, known respectively as the First (or Active) and the Second (or Passive).

a. The First Periphrastic Conjugation combines the Future Active Participle with the forms of sum, and denotes a future or intended action.

b. The Second Periphrastic Conjugation combines the Gerundive with the forms of sum, and denotes obligation, necessity, or propriety.

c. The periphrastic forms are inflected regularly throughout the Indicative and Subjunctive and in the Present and Perfect Infinitive.