# Who is weak, and I am not weak?

"Whenever anyone is weak, I feel that weakness also." or "I am weak whenever anyone else is weak." 

# Who has been caused to stumble, and I do not burn?

Paul uses this question to express his anger when a fellow believer is caused to sin. Here his anger is spoken of as a burning inside him. Alternate translation: "Whenever anyone causes a brother to sin, I am angry." 

# has been caused to stumble

"has been led to sin" or "has thought that God would permit him to sin because of something that someone else did" 

# I do not burn

"I am not angry about it"