It depends what the problems are, but if the exhaust emissions are out then it will fail. However - if it's an older car then it can make sense to get it MOT'd even if you know it will fail because at least that way you'll get a full list of all issues
eg no point fixing the engine if the MOT shows severe corrosion which is beyond economic repair.
If it goes through the MOT OK apart from the engine then you know it's worth sorting out.