Infinitive of a modal verb

An infinitive of a modal verb is conceptually absent from English grammar as full modal verbs lack infinitives; however modality can be expressed in an infinitive form by circumlocution. These are listed below:

Modal verb	Alternate positive infinitive forms	Alternative negative infinitive forms
can (preterite: could)	to be able to to be allowed to to be permitted to	to not be able to to not be allowed to to not be permitted to
will (preterite: would)	to be going to	to not be going to
may (preterite: might)	to possibly be going to to be allowed to	to possibly not be going to to not be allowed to
shall (preterite: should)	to be compelled to	to be compelled not to
must (preterite: must)	to have to to need to to be required to	to not be allowed to to not be permitted to