Wǒ wànɡle shuō zàijiàn le.
我忘了说再见了。
I forgot to say goodbye.
Notes:
我(wǒ): I
忘了(wànɡle): to forget
说(shuō): to say
再见(zàijiàn): goodbye
了(le): particle word
Example:
A:我忘了说再见了。
Wǒ wànɡle shuō zàijiàn le.
I forgot to say goodbye.
B:没关系。
Méiɡuānxì.
It doesn't matter.