Essentially, undergo means to experience. It can also mean to endure or suffer, which gives undergo a negative connotation.
Ex: My father will undergo surgery next week to repair the rotator cuff in his shoulder. We are hopeful that he will soon be pain-free.
Ex: Until I was in my last year of secondary school, I did not tell my mother that I had undergone bullying from two other girls in my class for three years.
Two words with go as their base sound the same but have very different meanings. Forego means to go or occur before the present time. In other words, forego means to precede. A foregone conclusion is an inevitability or a certainty. On the other hand, forgo means to do without.
Ex: Kurt is hopeless in athletic competitions, so it was a foregone conclusion that he would come in last in the cross country bike race.
Ex: Because it is almost Christmas, I must use what little money I have to buy gifts for my family and forgo purchasing the Ugg boots that I've wanted for years.
To grieve is to feel sorrow, generally after some significant loss. Grievous is the adjective form and describes something that causes pain or sorrow.
Ex: Most people grieve the loss of their youth and have low expectations for their futures. They soon discover the rewards of experience and look forward to aging.
Ex: Jack's grievous injuries resulted from his failure to wear a seatbelt. After three surgeries the doctors feel confident that he will recover completely.
Grief is the noun and is the feeling one experiences a great loss such as the death of a parent, or even the loss of a job. Good grief is an expression of dismay, surprise, or relief.
Ex: Jane was surprised at the grief she felt when she traded in her first car for a newer, better model. A lot of teenage memories were made in that old car, and she'd miss it.
Ex: Good grief! You've tracked mud all over my clean floor. How many times do I have to tell you to wipe your feet before coming into the house?