You weren't to know usage in British English. = depending on the situation. 'that's ok it wasn't your fault, you could not have known. Stop worrying about it, there are no hard feelings. It is a sort of cover all scenarios expression where someone cause something to happen and it wasn't their deliberate or careless fault, because they did not or were not told enough information, or were inexperienced not yet trained etc. You make an invited guest a meal and then they cannot eat it because of a deadly allergy you were not aware of, you feel awful, and the guest says "you weren't to know" Example that has happened to me. You delay and take your time closing a gate to an apparently empty field when all of a sudden there are hundreds of young bulls surrounding you and you can't close the gate because they are all pushing you in every direction. End result is the bulls escape onto a main road. In this situation I wasn't weren't to know that the empty field was not empty. You didn't have to know needs the full context to give accurate answer. Young bulls can charge up from the bottom of a slop where they can't be seen at 30mph and be on top of you in a few seconds whilst you back is turned whilst taking your time walking back a few yards to close a gate. It all ended with no casualties human or bovine.

As Dwayne said, "You weren't to know" is more commonly expressed as "You weren't *supposed* to know". However, "You weren't to know" is considered the subjunctive and is often used more in British English than in American English, though I *have* heard Americans use it too. Collins Dictionary says: You can say 'How was I to know?', or in British English 'I wasn't to know', to mean that you cannot be blamed or criticized for something you did, because you did not have enough information to realize that it was wrong. Examples: 'How was I to know you'd return so suddenly? You weren't to know–in fact, I think I'm the only person who does know.' https://www.collinsdictionary.com/dictionary/english/someone-wasnt-to-know-how-was-someone-to-know Longman Dictionary shows it's British English, as Irene said: be not to know British English used to say that you do not mind that someone has made a mistake because they could not have avoided it ‘Sorry, I didn’t realize you had guests.’ ‘That’s all right – you weren’t to know.’ 'The joint Chiefs were not to know.' https://www.ldoceonline.com/dictionary/be-not-to-know And Reverso dictionary says: It is also used as a subjunctive, esp. in conditional sentences https://dictionary.reverso.net/english-definition/you+weren%27t+to+know "You didn't have to know" means that it wasn't important for someone to know something or they weren't expected to have that information. "If I had known she was your ex girlfriend, I would not have invited her." "It wasn't important, you didn't have to know." But, "You weren't to know" can mean that someone else prevented you from having information or kept the information a secret so you would not find out. "Why didn't you tell me that my family was throwing a party for me?" "It was a surprise so you weren't to know." I hope this helps.

No, they are not the same. The first sentence would normally be used in a situation were somebody would have done something differently if they knew what ever it was, but they didn't know it and as a result something went wrong. Somebody might tell the person who made the mistake "you weren't to know" meaning "it wasn't your fault, you didn't know and couldn't have been expected to know" The second sentence means "there was no need for you to know'

No they are not. Your first sentence doesn’t really make sense. Maybe “you weren’t SUPPOSED to know”. Is a complete sentence. Your second sentence makes sense but if you provide the context I can better help you.