The document discusses different types of conditional sentences. It notes that first conditional sentences describe likely future events, using "if" and "will" to connect the clauses. Only present tense verbs can be used after "if", not future forms. The word "unless" can replace "if" with the meaning of "if not". Imperatives can also be used instead of clauses with "will". Time clauses with words like "when", "once", and "after" can also connect events.