What does macbeth mean when he says “ if it were done when ’tis done, then ’twere well“ in act 1 scene 7

Macbeth means that, if he could ensure that no complications arise from killing Duncan, it would be a good choice to kill him as soon as possible.

Explanation:

As Macbeth desires to become a king, he eventually kills king Duncan and ruins his own life, as he becomes consumed with guilt.

In Act I, Scene VII, there is a foreshadowing of Macbeth's tragic downfall. Macbeth predicts his own fate by saying that, if it was guaranteed that no consequences would follow Duncan's murder, he would kill him immediately. His tone changes when he continues by saying:  

"But in these cases

We still have judgment here"

Macbeth is aware that if he does kill the king, he will somehow be punished for his actions. He realizes that no good can come out of killing someone.

its structered sequentially i think