Which one is correct? Thanks.

Definitely the first one sounds more natural. I personally would use “the math class was interrupted by a smoke alarm (or fire alarm) this morning” but the first sentence sounds natural and gets the point across.

Both are fine, but personally, I would use no.1. You can only replace "ended" in the first example with "finished". It wouldn't work in the second example.

Both OK. Yes, you could use "finished" instead in #1. In #2, you'd need to rephrase it. For example, "The math class finished when the smoke alarms went off this morning." or "The math class finished due to the smoke alarms going off." In British English, it would be "maths" instead of "math".

Finish doesn't work because the class didn't finish. To finish it is to complete it. This class didn't reach its finish; it ended early because the fire alarm went off.

Both are fine. You wouldn't use "finish" because the classes didn't get to the end; they were interrupted.

Honestly, this one is tough. “Ended” is the culprit, as there are other words that more clearly communicate what happened. “The math class was interrupted by the smoke alarms this morning” is a better way to say this. To break it down, or for similar sentences, think of building a sentence from its most basic information first. Whatever is added to that sentence should answer questions for the person receiving the information (without them having to ask). Example: The class ended. - (What kind of class was it?) The math class ended. - (Was it supposed to end, or was this an unexpected occurrence?) The math class was ended. - (Why was the math class ended?) The math class was ended by the smoke alarms. - (When did this happen?) The math class was ended by the smoke alarms this morning.

People have covered the question you asked, but to add extra(probably irrelevant) context, in American English you'd say math, and in British English you'd say maths.

Finished not finish. would be acceptable.

The smoke alarms didn’t “end the class.” They caused the class to end. Likely the faculty member (teacher/professor/TA) said, “ok class is over,” after everyone was outside. It’s not “finished” because it ended early which means it didn’t finish. Otherwise we’d say, math class finished on time right when the smoke alarms went off/rang out.

They’re both correct. Americans use the active voice more often than the passive, so I recommend the first one. The phrase “was finished by” means something else: it gives a time at which something had already ended. For example, “The class was finished by two o’clock.”

I would say The math class ended early because of the smoke alarms this morning The math class was interrupted by the smoke alarms this morning. But of those two - the first one sounds the best.

No, the class didn’t finish, because it ended early.

FWIW we'd say fire alarm. Smoke alarms can be hushed/silenced, fire alarms need to be reset centrally by someone with a key and code to the panel.