The document discusses different types of conditional sentences in English:
- First type refers to possible and probable conditions using present or future tenses. Examples include "If you come I will be happy".
- Second type refers to possible but improbable conditions using past tense like "would". Examples include "If she studied she would pass her exams".
- Third type refers to impossible past conditions using past perfect tense like "would have". Examples include "If it hadn't rained we would have gone to the country".
The document also discusses expressions like "unless", "whether...or not", and "wish/if only" clauses.