When the clause has past time reference, the construction with the modal plus perfect infinitive ( see above ) is used : " If they ( had ) wanted to do it, they "'would ( could / might ) have done "'it by now . " ( The " would have done " construction is called the conditional perfect .)
12.
The use of " could " with the perfect infinitive expresses past ability or possibility, either in some counterfactual circumstance ( " I could have told him if I had seen him " ), or in some real circumstance where the act in question was not in fact realized : " I could have told him yesterday " ( but in fact I didn't ).
13.
Both " will " and " would " can be used with the perfect infinitive ( " will have ", " would have " ), either to form the future perfect and conditional perfect forms already referred to, or to express perfect aspect in their other meanings ( e . g . " there will have been an arrest order ", expressing strong probability ).
14.
Verbs that usually have a future reference, such as " swear ", " promise ", " hope ", " threaten ", " expect " etc ., either take the declarative infinitive ( mostly the future, but less often also the present, aorist or perfect infinitive, even the infinitive with representing a potential optative or indicative ), and in this case indirect discourse is employed, or they are followed by the " to "-infinitive ), as shown below.
15.
The voice remains unchanged; but the tense of the infinitive is mostly controlled by the temporal relationship between the time expressed by the matrix verb's tense and the time denoted by the infinitive : " Present " tense at the moment of utterance ( simultaneous state of affairs between the matrix verb and the infinitive ) is expressed by the Present Infinitive; " past " tense ( the infinitive's state of affairs is anterior to the time of the matrix verb ) by the Perfect infinitive; and " future " tense ( time posterior to the matix verb ) by the Future infinitive.
