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Do you have the link to the novel?
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You're not explaining the oath, so its confusing for someone who didn't read the novel.
Xiao Ling was "dating" Li Lei(she had dated him officially but after Li Lei's family tried to kill her they broke up, but kept seeing each other) when she went to film a commercial that included paragliding, but wind suddenly rises and she suffers an accident so li lei goes to her rescue(since he's an expert paraglider) and saves her from death but both get separated because of the wind and fall on different locations, Xia ling hears on the radio(that she cannot reach) that they know where she fell but not where Li Lei fall so she gets worried he'll die before they find him, so she takes a the ad's company cellphone and dials the only number she knows, Pei a personal line, a number only Xia Ling knew. Pei answers but ask her why she knew this number, she says Chu Chen told her but please to save Li Lei, Pei teoos her that he doesn't believe her and why would he save Li, so sje tells her the truth, that she's in fact Xia Ling and provenitnto him with some personal stuff from their past. Pei believes her but make her make an oath, she's to stay with Pei forever and forget about Li Lei, if she breaks the oath Li Lei dies. She accepts and Pei sends an helicopter for her and Li Lei, and she end up living with Pei. After recovering Xiao Ling finds that she's pregnant with Li Lei's child, worried that Pei wilo make her abort the child she tricks him by getting him drunk and making him believe that they had sex. And I could go on a on forever but that explain a few things.
I went to read the novel and the drama isn't small at all.
..... spoiler.....
1.) Xiao ling marries pei ziheng because of the oat she made with him
2.) Xiao ling child is born and pei ziheng knows it's not his and is not very happy about that
3.) Xiao ling child gets kidnapped during the party the threw for the kid and Xiao ling blames pei ziheng for it, which annoyes him and he trys to sleep with her so she can bear his own child. She refuses and stabs herself so she will never have a child for him.
4.) Pei ziheng tells Xiao ling that her child is dead while he is alive and was taken by her bother in her past life who knows who she is and that pei ziheng is not the father of the child but li lei
5.) Pei ziheng has a lot of work to do as li lei company is trying to bring his company down.
I hope it was helpful to anyone this is from the latest update on the novel, if you have any questions you can ask (=・ω・=).