Shall and will are both modal verbs primarily used to express the future tense.
Shall currently tends to be falling out of use, but it continues to be used
with I and we for offers and suggestions.
- Shall I open the window?
- Shall we dance?
Should is the conditional form of shall.
Would is the conditional form of will.
Will have or would have?
- We use the perfective will have when we are looking back from a point in time when something will have happened.
Ex: I will phone at six o’clock. He will have got home by then.
- or looking "back" from the present:
Ex: Look at the time. The match will have started.
- We use would have as the past tense form of will have:
Ex: I phoned at six o’clock. I knew he would have got home by then.
- We use would have in past conditionals to talk about something that did not happen:
Ex: He would have been very angry if he had seen you.