There are three types of conditional sentences in English: 1) First conditional - Used to refer to present or future situations that are possible or real. It uses the structure "if + present tense, will/present tense". 2) Second conditional - Used for unlikely, improbable, or hypothetical present or future situations. It uses the structure "if + past tense, would + infinitive". 3) Third conditional - Used for hypothetical or unlikely past situations. It uses the structure "if + past perfect, would have + past participle". The tenses are used to indicate the likelihood or time frame of the condition.