supposed to do

[Odessa Dawn]

They are supposed not to kill each other, some birds do.

It is supposed not to fight each other, some do.

We are supposed to learn others' cultures before their languages.

Does suppose to do usage in these sentences make sense? Can I used was/were to refer to the past?

[viettran]

I think they all make sense. And you can use was or were to refer to the past.

/not a teacher, though.

[HanibalII]

They are supposed not to kill each other, some birds do.

It is supposed not to fight each other, some do.

We are supposed to learn others' cultures before their languages.

Does suppose to do usage in these sentences make sense? Can I used was/were to refer to the past?

The usage is correct, but the placement is not.

'They are not supposed to kill each other, some birds do.'

The second sentence is incorrect. The word placement and word choice is very strange.

I would say 'They are not supposed to fight each other, some do'.

The third sentence is fine.

Not sure if it's a rule, but I think 'not' is to proceed 'supposed' in this type of context.

Yes, you can use was/were to refer to the past. Depending on context.

[probus]

They are all intelligible, but only the third is idiomatic. Here is an attempt to make the first two idiomatic.

Members of a species are not supposed to kill each other, but some birds do.

We are not supposed to fight each other but some people do.