He joined the
  congregation to pray Maghrib, and did not realize that the imaam was praying
    ‘Ishaa’
Question

                Assalaamu 'alaykum wa rahmatu 'Llahi wa
      barakatuhI prayed in congregation with the intention of making maghrib prayer;
      however, the congregation was praying the ishaa prayer because the imaam combined the two
      prayers. I didn't realise this until everyone stood for the forth rakat. How should I have
      corrected my salat ?  Jazzakallahu Khayrun
Praise be to Allah.If a person joins the imaam whilst he is praying
    Ishaa, but he has missed Maghrib for some legitimate reason, such as
    travelling, etc., then he should pray Maghrib even if the imaam is praying
    Ishaa, because the prayers have to be kept in the proper order. This is what
    the Prophet 
 (peace and blessings of Allaah be upon him) did during the campaign of
    al-Khandaq. Jaabir ibn Abd-Allaah said: Umar came on the day of al-Khandaq
    cursing the kuffaar of Quraysh and saying, O Messenger of Allaah, I did not pray
    Asr until the sun was almost setting. The Prophet 
 (peace and blessings of
    Allaah be upon him) said, By Allaah, I have not prayed it yet either. So he
    went down to But-haan [the name of a valley], did wudoo and prayed Asr after
    the sun had set, then he prayed Maghrib. (Reported by al-Bukhaari, 893)
    Al-Haafiz ibn Hajar said in al-Fath: This
    hadeeth teaches us how to make up missed prayers in order, and most scholars agree that
    this [keeping prayers in order] is obligatory.
Al-Bukhaari included this hadeeth in a chapter entitled Baab
    Qada al-Salawaat al-Oolaa fal-Oolaa (Chapter on making up prayers in
    order).
In the case mentioned in the question, it is permissible for you to
    pray Maghrib behind an imaam who is praying Ishaa, but when he gets up for the
    fourth rakah, you should remain sitting and recite the Tashahhud, and it is better
    to wait until he sits and says the Tasleem, so that you say the Tasleem with him. Then you
    should go ahead and pray Ishaa. And Allaah knows best.