In English grammar, third conditional sentences are used to describe hypothetical or unreal situations in the past and the possible outcomes of those situations. The third conditional typically takes the form of "if + past perfect, would have + past participle" as an example "If I had studied harder, I would have passed the exam."

A third conditional sentence consists of two parts: the condition (if clause) and the result (main clause). The condition is in the past perfect, and the result is in the past participle + "would have," "could have", or "might have". The past perfect form in the if clause indicates that the situation is hypothetical, and the past participle + "would have" or "could have" or "might have" in the main clause, indicates that the outcome is uncertain, hypothetical and unlikely.

Examples of third conditionals:

• If I had known about the traffic, I would have left earlier.
• If she had taken the train, she would have arrived on time.
• If it had rained, the streets would have been wet.

It's important to note that, like the first and second conditional, the third conditional also describes a cause-and-effect relationship, but the outcome is hypothetical, uncertain and unlikely in a past situation that did not happen. Third conditionals are often used to express regret or talk about past events or actions that did not happen, but that could have changed the outcome.

The third conditional is used to express hypothetical or unreal situations in the past, and it's formed with the past perfect in the if clause, and "would have", "could have", or "might have" followed by the past participle of the main verb in the main clause.

For example, "If I had studied harder, I would have passed the exam" is talking about a hypothetical situation where you studied harder and the outcome you would have passed the exam. Similarly, "If she had arrived on time, she would have met her friends" is expressing a hypothetical situation in the past where she arrived on time, and the outcome she would have met her friends.

It's also possible to use different modals verb in third conditionals, to express different degrees of probability or to express different attitudes. "I might have won, if I had tried harder" is expressing a lower degree of probability than "I would have won if I had tried harder."

It's important to remember that the third conditional is used to talk about hypothetical or unreal past situations when the speaker regrets or wishes that things had turned out differently. It's important to use the past perfect in the if clause, which refers to the hypothetical past situation and then use "would have", "could have", or "might have" in the main clause, which refers to the possible outcome that was not achieved.