Il aurait fallu, j'aurais mieux fait de, tu n'aurais pas dû

Il aurait fallu: nobody to blame

Falloir is impersonal, so il aurait fallu (it would have been necessary) names nobody at all.

• Il aurait fallu partir plus tôt. - We should have left earlier.
• Il aurait fallu le prévenir. - Somebody should have warned him.
• Il aurait fallu réserver. - We should have booked.

One thought, three levels of blame:

Name a subject and il aurait fallu takes que plus a subjunctive, as il faut does on the infinitive or subjunctive page:

• Il aurait fallu que tu partes plus tôt. - You should have left earlier.

At that point it stops being diplomatic.

J'aurais mieux fait de: the better choice

Faire mieux de means to do better to. In the conditional perfect it says another choice would have been the right one.

• J'aurais mieux fait de rester. - I would have done better to stay.
• Tu aurais mieux fait de te taire. - You would have done better to keep quiet.
• On aurait mieux fait de prendre le train. - We would have done better to take the train.

The linker is de, never à. Where j'aurais dû says an action was missed, this one names the better alternative, which makes it the phrase for a decision you regret.

Its impersonal twin is valoir mieux:

• Il aurait mieux valu attendre. - It would have been better to wait.

Tu n'aurais pas dû: reproach or thank-you

The negative wraps the auxiliary, as in any compound tense: je n'aurais pas dû, tu n'aurais jamais dû.

• Tu n'aurais pas dû lui dire ça. - You shouldn't have said that to him. (a reproach)
• Elle n'aurait jamais dû accepter. - She should never have accepted.
• Oh, tu n'aurais pas dû ! - Oh, you shouldn't have! (unwrapping a present: a thank-you)

Only tone and moment tell the first and the last apart. Have the thank-you ready: being handed a gift and saying nothing is worse than saying something slightly wrong.

Mistakes English speakers make