Periphrastic verbs are multiword alternatives to other, dfective, 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.
So, while I can't say *I'd like to can swim as can has no infinitive form, I can replace it with be able to : I'd like to be able to swim.
