Perfect tenses, as perfect suggests, express the completion of verbal action. So quite similarly we have to look for auxiliary verbs and form of the main verb that show completion of verbal action.
Auxiliaries that show completion of verbal action
|Present form (used in present tenses)||3rd person singular form (used in present tenses with third person singular pronouns, like; he, she, it and singular noun)||Past form (used in past tenses)|
The use of have in present perfect tense is associated with I, we, you, they and plural noun as subjects, whereas for all others its 3rd person singular form [has] shall be used. In past time situations [had] is used with all subjects.
- He has informed me about seriousness of the matter.
- You have won his sympathies.
- They had come in time.
For future time situations modal operators (will and shall) are used along with have (the present form of auxiliary).
- They will have departed.
- I shall have checked the mail.
Future perfect tense is not always used for future time situations. There are certain conditions in which it is used quite other way round. It shows probability of verbal action in past time situation. Look at the following examples;
- They will have departed by now.
In above sentence the situation is not future, rather it talks about the past and the verbal action seems to have been done already. The use of will in this case gives an impression of some sort of probability and it too is quite possible that situation may turn out to be quite otherwise.
The use of shall is quite restricted and generally it is replaced by will as you might have come across many such phrases like; I will (I'll), we will (we’ll) etc. Interchange of shall and will also indicates emphasis on verbal action, like;
- They will have shifted. (no emphasis)
- They shall have shifted. (emphasis)
It is due to the same while writing contract or in official documents will is mostly replaced by shall, like;
- It shall be considered final.
- Arrangements shall be made.
- Fourth of July shall be the holiday.
For more used of shall go to the chapter of simple tenses.