I. Infinitive Basics
When you look up a Latin verb in a Latin-English lexicon or dictionary, you will see 4 entries (principal parts) for most verbs. The 2nd entry, usually abbreviated "-are," "-ere" or "-ire," is the infinitive. More specifically, it's the present active infinitive, which is translated into English as "to" plus whatever the verb means. The vowel (a, e, or i) of the infinitive indicates which conjugation it belongs to.
Example of a dictionary entry for a verb in Latin:
Laudo, -are, -avi, -atus. Praise
The 1st entry in the dictionary entry is the present, active, singular, 1st person form of the verb. Note the -o ending. Laudo 'I praise' is a first conjugation verb and therefore has an infinitive ending in "-are." The entire present active infinitive of laudo is laudare, which we translate into English as "to praise."
Laudari is the present passive infinitive of laudo and means "to be praised."
Most verbs have 6 infinitives:
Infinitives have tense and voice.
- Present Active
- Present Passive
- Perfect Active
- Perfect Passive
- Future Active
- Future Passive (rare)
Perfect Infinitives of Latin Verbs
The perfect active infinitive is formed from the perfect stem. In our example of a 1st conjugation verb, laudo, the perfect stem is found on the third principal part, laudavi, which is listed in the dictionary simply as "-avi." Remove the personal ending ("i") and add "isse" -- laudavisse -- to make the perfect active infinitive.
The perfect passive infinitive is formed from the fourth principal part, in our example, laudatus, plus "esse." The perfect passive infinitive is laudatus esse.
Future Infinitives of Latin Verbs
The fourth principal part also informs the future infinitives. The future active infinitive is laudaturus esse and future passive infinitive is laudatum iri.
Infinitives of a 1st Conjugation Latin Verb
- (Present Active) amare (love)
- (Pres. Passive) amari
- (Perf. Act.) amavisse
- (Perf. Pass.) amatus esse
- (Fut. Act.) amaturus esse
- (Fut. Pass.) amatum iri
Infinitives of a 2nd Conjugation Latin Verb
- monere (warn)
- monitus esse
- moniturus esse
- monitum iri
Infinitives of a 3rd Conjugation Latin Verb
- regere (rule)
- rectus esse
- recturus esse
- rectum iri
Infinitives of a 4th Conjugation Latin Verb
- audire (hear)
- auditus esse
- auditurus esse
- auditum iri
II. More Details on the Latin Infinitive
On Interpreting the Infinitive
Although it may be easy enough to translate the infinitive as "to" plus whatever the verb is (plus whatever person and tense markers may be required), it can be hard to explain the infinitive. It acts like a verbal noun, for which reason, it is sometimes taught alongside the gerund.
Latin Infinitives in Indirect Statements
Latin Composition's Bernard M. Allen says that 3/8 of the times that an infinitive is used in Latin, it is in an indirect statement. An example of an indirect statement is: "She says that she is tall." In Latin, the "that" wouldn't be there. Instead, the construction would involve a regular statement, she says (dicit), followed by the indirect part, with the subject "she" in the accusative case followed by the present infinitive (esse):
Dicit eam esse altam.
She says (that) she [acc.] is [infinitive] tall [acc.].
Allen says that Bennett's Grammar provides a rule for the tense of the infinitive that is only applicable to the present infinitive in indirect statement. Bennett's rule is:
"'The Present Infinitive represents an act as contemporaneous with that of the verb on which it depends.'"
Bernard prefers the following:
"'In Indirect Statements the present infinitive represents an act as contemporaneous with the time of verb on which it depends. In other substantive uses it is merely a verbal noun, without any tense force.'"
Tense in Latin Complementary Infinitives
As an example of why tense is a difficult concept with present infinitives, Allen says that in Cicero and Caesar, 1/3 of their present infinitives follow the verb possum 'to be able'. If you are able to do something, that ability precedes the time of the statement.
Other Uses of the Infinitive
An infinitive can also be used as the subject of a sentence. The subjective infinitive is found after impersonal expressions like necesse est 'it is necessary'.
Necesse est dormire.
it is necessary to sleep.
"The Latin Present Infinitive," by Bernard M. Allen. The Classical Journal, Vol. 19, No. 4 (Jan., 1924), pp. 222-225