Because the sentence focuses on the recent activity itself and its visible result. The activity (painting) is emphasized, and the evidence of that activity (paint all over the floor) is still visible. We don't know whether the painting activity is finished and it doesn't matter. 

Because the room was painted a long time ago and they are talking about a past event. 

Because the action is finished and the result (a freshly painted room) is the main focus. We understand in the sentence that the action is completed, and that's why there is paint all over the floor. 

have you been living 

you lived

did you live

have you lived 

have you been lived 

has answered 

has been answering 

answered 

was answering 

has been going

went 

have been going

has going 

has gone

I must finish this book tonight.

I don't have finish this book tonight.

I should finish this book tonight.

I have to finish this book tonight. 

I could finish this book tonight

did you do

have you doing 

has you done

have you been doing

He must be tired — he hasn’t slept. 

He should be tired — he hasn’t slept.

He might be tired — he hasn’t slept.

He could be tired — he hasn’t slept.

 Because the sentence describes a temporary activity that has been happening recently and is not finished.

Because it refers to a completed action in the past with no connection to the present.

Because it describes a state (being a teacher) that started in the past and continues up to now. 

could

must

can

may

should

May your dreams come true

I may have left my key at the office.

May I borrow your pen?