Conditional sentences describe situations and their possible or imagined results. The zero conditional expresses general truths using "if + present simple, present simple," as in "If you heat water to 100°C, it boils." The first conditional addresses real or likely future situations with "if + present simple, will + base verb," such as "If it rains, I will stay home." The second conditional describes unlikely or hypothetical present/future situations using "if + simple past, would + base verb," like "If I won the lottery, I would travel the world."
The third conditional explores unreal past situations and imagined results using "if + past perfect, would have + past participle," as in "If I had studied, I would have passed." A common error in the third conditional is using "would have" in the if-clause ("If I would have known"), which is incorrect; only "had + past participle" appears there. Mixed conditionals combine past and present: an if-clause with past perfect paired with a main clause in present conditional ("If I had studied medicine, I would be a doctor now").
The verb "wish" expresses regrets and unreal situations. For present wishes, use "wish + past simple" ("I wish I had more time") or "wish + could + verb" for abilities ("I wish I could fly"). For regret about a past action, use "wish + past perfect," as in "I wish I had studied harder." The expression "if only" functions similarly: "If only I were taller" uses the past simple for present regrets, paralleling the second conditional.