The document discusses different types of conditional sentences in English:
1. Zero conditional sentences use the present tense to describe general truths.
2. Type 1 conditional sentences use the present tense and will/imperative to talk about possible events in the future.
3. Type 2 conditional sentences use the past tense and would to talk about unlikely or impossible events.
4. Type 3 conditional sentences use the past perfect tense and would have to speculate about past events and how things might have been different.
5. Mixed conditional sentences use both past perfect and would/would have to discuss past events that may still have present consequences or present facts that explain past events.