An ELT Glossary : Periphrastic verbs

Periphrastic verbs are phrase-like alternatives to other single word verbs. Most of the modal verbs have a periphrastic alternative which can be used when the sentence requires a form which the modal itself lacks - eg the infinitive or a participle.

For example, the modal can has the periphrastic alternative be able to, while will is complemented by be going to.