how to use the word "unless" in a sentence?

Dear friends and teachers:

I hope you all well. I have encountered one question when i was writting a short reply. The queston was how to use the word " unless" to make a sentence?

Can i say " i wouldnot catch a bus unless when i were in Australia." or should i say " i will not catch a bus unless when i am in Australia" ? I am confused about what tense should i use if my intention here is to express an assumption ( conditional probability maybe )

As it shows i am a grammar idiot, so please help me~~ Thanks in advance!

Webreaper wrote:

Neither is correct, really. The way you'd structure it would be "I wouldn't catch a bus unless I was in Australia".  You could also use "I wouldn't catch a bus except if I were in Australia", but it doesn't scan as nicely.

Do u mean " unless when..." is not a native expression ? And I made the sentence above merely based on the example " i wouldnot do it if i were you", so Webreaper would you please tell me the difference bewteen " i was in Australia" and " i were in Australia"?

thx in advance