Conditional sentences, also known as if-then sentences, are sentences that express hypothetical situations and their possible outcomes. They typically consist of two clauses: the "if" clause (or condition clause) and the "then" clause (or result clause). Conditional sentences often use the word "if" to introduce the condition and express a potential consequence or result.

There are several types of conditional sentences, including:

1. Zero conditional: Used to express general truths, facts, or habits. It typically uses the present simple tense in both clauses. For example: "If you heat water, it boils."

2. First conditional: Used to talk about real possibilities in the present or future. It typically uses the present simple tense in the if-clause and the future simple tense (with "will") in the result clause. For example: "If it rains tomorrow, I will stay indoors."

3. Second conditional: Used to talk about unreal or unlikely situations in the present or future. It typically uses the past simple tense in the if-clause and the conditional tense (with "would") in the result clause. For example: "If I won the lottery, I would travel the world."

4. Third conditional: Used to talk about unreal situations in the past and their hypothetical outcomes. It typically uses the past perfect tense in the if-clause and the conditional perfect tense (with "would have") in the result clause. For example: "If she had studied harder, she would have passed the exam."

Conditional sentences are important for expressing possibilities, hypothetical situations, and potential consequences in both spoken and written English.