He swore that he would stop watching movies, then he watched a movie by mistake. Does he have to offer expiation, and how can he stop watching movies?
Question

                I swore that I would not watch movies, but I did not specify any particular type of movies. One year later I watched a movie in which there was nothing bad or indecent, and I forgot my vow. (I thought that I was not watching a bad movie). Is there any expiation for that? With regard to the future, is breaking a vow a major sin? How can I stop committing this sin?.
Praise be to Allah.
    Firstly: 

    Movies are not free of a number of haraam things, such as 
    women displaying their adornments, music, and so on. We have previously 
    discussed the ruling on all movies, including a number of the haraam things 
    they contain, in the answers to several questions. See the answers to 
    questions no. 72204,
    85232,
    125535 and
    114707. 

    If the movies are indecent or permissive, then the 
    prohibition on watching them is even more emphatic. 

    For more information please see the answer to question no.
    12301. 

    Secondly: 

    Your vow not to watch movies includes all kinds of movies, so 
    long as you did not intend anything specific. 

    Based on that, it is not permissible for you to watch any 
    movie and if you go against that then you have to offer expiation for 
    breaking an oath (kafaarat yameen). 

    The sin that is involved in watching movies applies whether 
    one vowed not to watch them or not. 

    But, with regard to the expiation for breaking an oath, this 
    expiation is not required if at the time of watching it one had forgotten 
    the oath. 

    To find out more about the expiation for breaking an oath and 
    what it entails, see the answers to questions no.
    9985 and
    9943. 

    Ibn Qudaamah (may Allah have mercy on him) said: 

    Whoever swears that he will not do something, then he does it 
    by mistake (because he forgot) does not have to offer expiation for it. A 
    number of scholars narrated this from Ahmad. This applies except in the case 
    of divorce (talaaq) and manumission, in which case it is regarded as 
    breaking the oath. This seems to be the view of the madhhab and it is the 
    view favoured by al-Khallaal and his companion. It is also the view of Abu 
    ‘Ubayd and of al-Shaafa‘i, because of the words of Allah (interpretation of 
    the meaning): “And there is no sin on you 
    concerning that in which you made a mistake, except in regard to what your 
    hearts deliberately intend” [al-Ahzaab 33:5]. And the Prophet 
    (blessings and peace of Allah be upon him) said: “Allah has forgiven my 
    ummah for mistakes, what they forget and what they are forced to do.” And 
    because he did not intend to go against it, so he has not broken his vow. 
    This is like one who is asleep or one who is insane. End quote. 

    Al-Mughni Sharh Mukhtasar al-Khuraqi, 
    9/391. 

    Shaykh Muhammad ibn Saalih al-‘Uthaymeen (may Allah have 
    mercy on him) said: 

    If someone says, By Allah, I will not read this book today, 
    then he forgets and reads it – he does not have to do anything, because he 
    forgot. But if he has no excuse (for breaking his oath), such as being 
    compelled or forgetting, then he has to stop as soon as that excuse no 
    longer applies, otherwise he has broken his oath. End quote.

    Al-Sharh al-Mumti‘ ‘ala Zaad al-Mustaqni‘, 
    15/137-138 

    See also the answer to question no.
    42334 

    Thirdly: 

    With regard to ways of stopping watching movies, you can 
    achieve that by means of things that have been mentioned by the scholars and 
    educators. For details on that please see the answer to question no.
    159366. 

    We ask Allah to help you to do that which He loves and which 
    pleases Him. 

    And Allah knows best.