Infinitive of a modal verb

From Teflpedia

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 (past tense: 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 (past tense: would)
  • to be going to
  • to not be going to
may (past tense: might)
  • to possibly be going to
  • to be allowed to
  • to possibly not be going to
  • to not be allowed to
shall (past tense: should) to be compelled to
must
  • to not be allowed to
  • to not be permitted to