What are the functions of each part of the principal parts?

Solution 1:

The finite forms

• present An inflected form with -s is used for the 3rd person singular. Be has distinct forms for both the 1st and 3rd persons singular.

  • Used to express states which are currently true and actions habitually performed in the present; may also express intended future actions or states.

• past (formed with the -(e)d suffix in weak verbs, with vowel change in strong verbs, and variously in irregular verbs. Be has a distinct form for the 1st and 3rd persons singular.)

  • Used to express states which were true at a given point in the past and actions completed in the past.
  • Used without personal inflection in the protasis of some conditional clauses, when it is called the past subjunctive in traditional grammar.

The non-finite forms
(modal verbs are defective: they have no non-finite forms and cannot serve the following uses)

• infinitive (for all verbs except be identical with the present form) 

  • Used unmarked (‘bare infinitive’) in constructions with modal verbs (can/could, may/might, shall/should, will/would, &c) to express modality and futurity
  • Used marked with to (‘marked infinitive’) in constructions with semi-modal verbs (dare, ought, &c) to express modality
  • Used marked with to, with and without a subject, in subordinate and complementary clauses
  • Used in certain subordinate clauses expressing an action commanded to be performed, when it is called the present subjunctive in traditional grammar.

• past participle (identical with the past form in weak verbs; formed with vowel change, and sometimes an -(e)n suffix in strong verbs; formed variously in irregular verbs)

  • Used in constructions with be to express the passive voice
  • Used in ‘perfect’ constructions with have to mark events as occurring before reference time but still having relevance at the reference point
  • Used as an adjective to express having suffered an action (if transitive: a marked man) or having performed an action (if intransitive: a traveled woman)

• present participle (formed with the -ing *suffix)

  • Used with be to express progressive (continuing) aspect
  • Used as an adjective to express current performance of an action at reference time
  • Used as a ‘gerund’ capable of occupying the syntactic role of a noun