There are generally four types of conditional sentences in English, each serving a different purpose. The zero conditional (using the present simple in both clauses) describes general truths or laws of nature, like "If water reaches 100 degrees Celsius, it boils." The first conditional, as mentioned, is used for real or possible situations. The second conditional (using the past simple with "would" plus the base form of the verb) is for hypothetical situations in the present or future. Lastly, the third conditional (using "had" with "would have" plus the past participle) talks about hypothetical situations in the past, situations that did not happen, like "If I had known about the sale, I would have gone shopping." Each type serves a unique purpose and is used based on the likelihood and time frame of the situation being discussed.