时态问题1.they had paid for expensive seats at the theatre,only

时态问题1.they had paid for expensive seats at the theatre,only to find the show is disappointing.
2.they set out for the ariport at very early time,only to find that flight had been canceled.
为什么only to find 后面的句子时态不一样呢?怎么分析这两个句子呢?
另外,分析句子的时态有什么窍门吗?