The zero conditional expresses certainty, stating facts that are always true. Both the condition and the result use the present tense, and "when" can replace "if." Examples of this tense are "If you don't drink enough, you become dehydrated" and "When you don't drink, you become dehydrated."
The first conditional expresses a real possibility of something happening in the future. The condition is in the present simple, such as "you walk." The result uses "will" plus a verb in base form, such as "you will find." An example of the first conditional could be "If you walk down that street, you will find the store" or the reverse, "You will find the store if you walk down that street." In some cases, "shall," "can" or "may" can replace "will."
The second conditional speaks of an unreal situation in the future. Both the condition and the result in this conditional are very unlikely, although not impossible. The condition is in the past simple tense, such as "he ran." The result uses "would" plus a verb in base form, such as "would not be late." This creates a conditional sentence of "If he ran to work, he wouldn't be late." Substitutes for "would" can be "should," "could" or "might."
The third conditional refers to an impossible situation. Based in the past, it talks about the result of something that didn't happen. The condition is in the past perfect tense, such as "the car had broken down." The result uses "would have" plus the past participle, such as "would have stayed." An example could be "If the car had broken down, we would have stayed at home" or a question such as "What would you have done if he hadn't arrived?" Alternatives for "would have" include "should have," "could have" and "might have."