This document discusses conditional sentences in English. It begins by defining conditional sentences as having two clauses - an 'if clause' and a main clause that are closely related, with one part showing a result and the other a condition. It then lists some common modal verbs used in conditional sentences like must, can, may, will, shall, ought, could, might, would, and should. Finally, it outlines the three types of conditional sentences - probable, improbable, and impossible conditions - providing examples and explaining the nature, time, structure, and modal verbs used for each type.