The document describes different types of conditional sentences in English:
1) First conditional sentences use "if + present tense" in the first clause and "will + base form of the verb" in the second clause to talk about possible or likely future events. 
2) Second conditional sentences use "if + past tense" in the first clause and "would + base form of the verb" in the second clause to talk about unlikely or hypothetical future events. 
3) Conditional sentences with "if + subject + were" follow the same structure as second conditional sentences but use "were" instead of the past tense form of the verb "to be" for all subjects.