〜さえ・〜でさえ

さえ means "even", picking out an extreme example to make a point: 忙しくて、ご飯を食べる時間さえない, "I'm so busy I don't even have time to eat". If even that is true, everything else surely is.

It usually replaces が and を. Other particles stay in front: 親友にさえ話していない, "I haven't told even my best friend".

でさえ is common with people: 子どもでさえ知っている, "even a child knows that".

It's stronger than も (時間もない) and more formal than でも. In writing, すら means the same. With a negative, it's often "not even", and with ば it becomes "as long as", which is the next point.