"Be supposed to" can mean "they say".

- It's supposed to be a barbecue
= They say it's going to be a barbecue
- It's supposed to rain later
= They say it's going to rain

"Be supposed" to can mean "have to" or "should".

- I'm supposed to work tomorrow
= I should work tomorrow
- He's not supposed to eat chocolate
= He can not eat chocolate

"Be supposed to" can contrast what should happen with what does or will happen.

- I'm supposed to be studying for an exam, but I'm not
- I'm not supposed to stay out late, but maybe I will

"Was/were supposed" to can mean what was expected didn't or won't happen.

- They were supposed to come at seven (but probably they didn't)
- I wasn't supposed to go by myself (but I'm going)

"Was/were supposed to" can mean "intended to" (wanted to, had that intention).

- He was going to give us directions, but he didn't
- I wasn't going to go to the party, but I guess I will