Conditional Sentence

 Conditional sentence
referred to as sentence modality, because of the meaning contained in the sentence is contrary to the true meaning. In English there are three types of conditional sentences, which each have their own usefulness. In a sentence modality, there are two clause, namely:
1. supposition clause, often referred to as the “if clause”
2. expectations of the clause, which in this case as the “main clause”

type1.
Used to express the things that diaharapkan would happen, for describing the things that will happen. Thus, this sentence describes a certain thing will happen, if at certain things are met.
If + subject + V1 + object: subject + will + V1
contoh:
menggunakan kata kerja : - if he gives me an/the invitation, I will come to the party.
                                        -I believe that he will be angry with me if I come late.
tidak menggunakan kata kerja: -You will pass your test if you study hard.
                                               -If he has many problems, I will help him.

Type 2
used to express things that are actually contrary to the situation now, not to predict things that will happen.
If + subject + V2 + object: subject + would + V1 + object
contoh:
1.      if I were a teacher, I would teach them. (seandainya saya seorang guru, saya akan mengajarkan mereka)
2.       what would you do if you were her brother? ( apa yang akan kamu lakukan, seandainya kamu kakaknya?)
3.       If he gave the letter, I could read it. (seandainya ia memberikan surat itu, saya dapat membacanya).
4.       If he were In the room, he could see the picture. (seandainya ia di ruangan itu, ia dapat melihat gambar itu).

Type 3
used to express the things that had already happened, the impossible will happen again.
If + subject + had + V3 + object: I + would have + V3 + object
cth:
1.       if he had invited me, I could have come to the party. (seandainya ia telah mengundang saya, maka saya datang ke pesta itu)>
2.       if she had known the news, I believe that she would have gone. ( seandainya ia telah mengetahui berita itu, saya yakin bahwa ia sudah pergi).
3.       If he had been there, I could have told him the matter. (seandainya ia telah ada di sana, sebenarnya saya sudah dapat mengatakan hal itu kepadanya).

4. If we had known that yaur were there, we would have written yaou letter.

Penggunaan :
 Tipe I : menggambarkan tentang mengandai-andai sesuai dengan fakta. 
 Tipe II :menggambarkan tentang pengandaian yang tidak nyata atau berimajinasi. 
 Tipe III : digunakan sebagai penyesalan yang terjadi di masa lampau

sumber : http://maulanajayadi24hikaru.blogspot.com/2011/11/conditional-sentence.html