Conditional tense is used to express conditional statements involving real, imaginary, or past situations. There are three types of conditional sentences: Type 1 uses "if + present tense" and "present future" to discuss real possibilities; Type 2 uses "if + past tense" and "would + verb" to discuss imaginary situations; Type 3 uses "if + past perfect" and "would have + past participle" to discuss regrets about the past. Examples are provided for each type, along with explanations of changing conditionals from positive to negative and vice versa.