¿Cuál es la diferencia entre unnecesary y needless ?Puedes dar oraciones como ejemplo.

That's actually quite a hard question! I had to think about it quite a lot so I hope I can make it clear.

They are basically interchangeable in someways and very similar in meaning:

A needless waste of time/an unecessary waste of time

A needless expense/an unecessary expense

I would use unecessary more often but I think that needless perhaps has a stronger sense of disapproval or negativity. Needless is used in phrases like:

Needless violence/needless loss of life/needless suffering

You can still use unecessary here but I think needless is more common and somehow feels a bit stronger.

On the other hand, I think unecessary has more of a sense of surplus to requirements:

They're building an unecessary airport/The airport is totally unecessary

In those sentences, you can't exchange needless.

They are very close so it's a little difficult to explain, I hope that's helped and hopefully you will get some other opinions that may help to make it clearer!