買います in casual form is 買う.
買わなかった is 買う in past negative.
To change a tybe 1 verb to negative form, you change the last う-kana to the corresponding あ-kana and add ない in the end. Note that う is changed to わ instead of あ. So now it is 買わない.
To make it past tense you change the い to かった, so it becomes 買わなかった.

The meaning of 「高いから買わなかった」 is “it’s expensive, so I didn’t buy it.”

I hope that answered your question.

“To die” in dictionary form is 死ぬ.
To say that you want to do something, you convert it into formal form, and replace the ます with たい. So, 死ぬ -> 死にます -> 死にたい. Which means “want to die.” Eew, I don’t like this one.
Now it is actually an い-adective. So, if something is 死にたい, it wants to die. You turn it into negative form the same way as you turn any other い-adjective negative; you replace the い in the ending with ない. So now it is 死にたくない. And から means roughly “because.”

This one was a little tough because you had to turn it formal first.