If you heat water, it boils. This is a simple conditional sentence referring to a general truth using the present tense. If you don't write it down, you will forget. This is a first conditional sentence referring to a probable future event using the present tense in the if-clause and will + bare infinitive in the main clause. If I were you, I would write the business plan. This is a second conditional sentence referring to an improbable future event using the past tense in the if-clause and would + bare infinitive in the main clause. If I had been at the party, I would have met her. This is a third conditional sentence referring to an impossible past event