In English language there are six infinitives with transitive verbs and four with intransitive.
Infinitive forms of a transitive verb
|Pres. simple||to form||to be formed|
|Pres. progressive||to be forming||/|
|Perfect simple||to have formed||to have been formed|
|Perf. progressive||to have been forming||/|
Infinitive forms of an intransitive verb
|Pres. simple||to sit||/|
|Pres. progressive||to be sitting||/|
|Perfect simple||to have sit||/|
|Perf. progressive||to have been sitting||/|
An infinitive may be used with or without the particle to.
An infinitive without to is known as the bare or plain infinitive