■IF 現在形　＋　主節現在（実現可能性100%）
If you accept our offer, we have a deal.

■IF 現在形　＋　主節助動詞
If you accept our offer, we will have a deal.（実現可能性80%）
If you accept our offer, we may have a deal.（実現可能性60%）
If you accept our offer, we must have a deal.（実現可能性110%）

＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝

Subjunctive mood

■IF 過去形　＋　主節過去の助動詞
If you accepted our offer, we would have a deal.（実現可能性0%～）
If you accepted our offer, we should have a deal.
If you accepted our offer, we could have a deal.
If you accepted our offer, we might have a deal.
現在の願望を表す。するのになぁ。

■IF 過去完了形　＋　主節過去の助動詞＋完了形
If you accepted our offer, we would have had a deal.したのになぁ（100%後悔）
If you accepted our offer, we should have had a deal.すべきだったのになぁ（200%後悔）
If you accepted our offer, we could have had a deal.できたのになぁ（60%実感）
If you accepted our offer, we might have had a deal.したかもなぁ（35%関心薄）
過去の願望を表す。したのになぁ。