The first one isn't completely wrong, but it would be unusual and awkward. I'm not sure exactly what you are trying to say, but here are some example sentences: The project was stressful for them. (adjective) <-- very common The project created/meant/was a lot of stress for them. (noun) It / the cause of his health problems was cancer. (noun) The result of the test was: cancer (noun) He has/had cancer. (noun) His lung/tumor/etc was cancerous. (adjective) I hope that helps!

Good question. We do not usually say “It was stress (for them)” because “stress” describes a condition or feeling, not something that clearly identifies what “it” refers to. The sentence feels incomplete without context. You could say “It was stressful for them” or “They were under a lot of stress.” “It was cancer” works because “cancer” is a specific disease that clearly identifies what “it” refers to. For example, if someone is talking about an illness and says “It was cancer,” that fully explains the situation. To summarize, “It was stress” sounds unnatural because “stress” does not identify a specific event or cause the way “cancer” does. You need to describe it differently, such as “They were under stress” or “It was a stressful time.”