The basic idea: one construction, two time references
In the present, be not supposed to can express a current prohibition or an expectation that an action will not happen. Was/were not supposed to refers to the corresponding past rule or expectation. What actually happens may or may not match it.
Be locates the rule or expectation in time, not necessarily a completed action: Yesterday I wasn't supposed to work today means that yesterday's plan did not require me to work today. To report a violation, state it explicitly.
Tenants are not supposed to hang laundry on the balcony rail.
Tenants are not supposed to hang laundry on the balcony rail.
A current prohibition in the rental agreement, in force now.
The results weren't supposed to be published before Monday, yet a summary leaked on Friday.
The results weren't supposed to be published before Monday, yet a summary leaked on Friday.
This concerns a past event: the expectation has been violated.
Form: negation, agreement, and the base verb
Not goes immediately after be: is not / are not / was not / were not, or the contractions isn't / aren't / wasn't / weren't. Supposed to is followed by a base verb, not an -ing form, and to must not be omitted.
In the present: I am not, he/she/it is not, you/we/they are not. In the past: I/he/she/it was not, you/we/they were not. You requires are/were even when addressing one person.
He isn't supposed to skip the warm-up before training.
He isn't supposed to skip the warm-up before training.
One person in the present → isn't, with the base verb skip.
We weren't supposed to reschedule the call twice.
We weren't supposed to reschedule the call twice.
A plural subject in the past → weren't, not wasn't.
Meaning 1: not supposed to — a current prohibition or expectation
The present form can describe a general rule or a specific arrangement: You're not supposed to call her tonight. By itself, it does not tell us whether the rule has already been broken.
Identify the meaning from the situation, not from the absence of yesterday or but. A current rule can combine with a past violation: We're not supposed to park here, but I did yesterday.
Volunteers are not supposed to touch the artefacts without gloves.
Volunteers are not supposed to touch the artefacts without gloves.
A current museum rule that applies now.
The app isn't supposed to save passwords locally, according to the new policy.
The app isn't supposed to save passwords locally, according to the new policy.
This concerns how the app should behave from now on, rather than a single incident.
A past prohibition: was it followed or broken?
Wasn't/weren't supposed to is often used when recounting a violation. However, the form alone does not prove one: I wasn't supposed to tell anyone, so I kept quiet means that the person obeyed the prohibition.
But she did it or yet they began explicitly reports a violation. Without such a continuation or relevant context, the outcome remains open.
The assistant wasn't supposed to forward the client list, but she did it by mistake.
The assistant wasn't supposed to forward the client list, but she did it by mistake.
The action has happened and violated the requirement in force at the time.
We weren't supposed to start the meeting without the director, yet the team began without her.
We weren't supposed to start the meeting without the director, yet the team began without her.
A deliberate or accidental failure to follow a past requirement.
Meaning 2b: wasn't/weren't supposed to — a plan or prediction that did not hold
The past form can also express a prediction or plan that did not work out. This need not concern a prohibition: a parcel was expected not to exceed a certain weight, but it turned out to be heavier.
The grammatical form does not establish whether someone made a mistake or was responsible. An inanimate subject does not prove that no one was at fault either. First distinguish a requirement from a prediction, then consider the facts of the situation.
The parcel wasn't supposed to weigh more than two kilos, but the scale showed three.
The parcel wasn't supposed to weigh more than two kilos, but the scale showed three.
The expected weight differed from the measured weight. The sentence does not explain who is responsible for the discrepancy.
The bridge repairs weren't supposed to affect traffic, yet the whole street was blocked for a day.
The bridge repairs weren't supposed to affect traffic, yet the whole street was blocked for a day.
This states a mismatch between the plan and the repairs' effects; it does not identify causes or responsibility.
Discourse: but/yet clauses and questions that may express criticism
But or yet conveniently introduces what happened contrary to the expectation. A continuation is optional: the past form can simply recall a rule already known to the listener.
Wasn't/Weren't … supposed to…? can express criticism, surprise, or an ordinary memory check. Intonation and context show the intention; the question form alone does not prove that the obligation was unfulfilled.
Weren't you supposed to confirm the booking last night?
Weren't you supposed to confirm the booking last night?
This may be criticism or a check of the arrangement. The answer tells us whether the booking was confirmed.
The lift wasn't supposed to stop between floors, but it did twice this week.
The lift wasn't supposed to stop between floors, but it did twice this week.
The but-clause states what actually happened contrary to the expectation.
Scope: what this rule does not cover
The affirmative be supposed to, without not, describes expected behavior or a rule and belongs to the neighboring rule Expected behaviour or rules. Here it appears only for comparison, not as the exercise target.
Be meant to is a synonymous construction with a somewhat different emphasis on intention or purpose, and is more characteristic of British usage. It belongs to the separate rule Be meant to as a synonym and appears here only for comparison.
Neither evaluations of past actions with should have / shouldn't have nor hypothetical constructions such as the third conditional are covered here; they are different structures in other topics in the Advanced Modals category.
Guests are supposed to arrive by seven.
Guests are supposed to arrive by seven.
Affirmative be supposed to belongs to a neighboring rule and is included here only for comparison.
This device is meant to filter fine dust, not just large particles.
This device is meant to filter fine dust, not just large particles.
Be meant to is a synonymous but separate construction, shown only to distinguish it.
Common mistakes
Incorrect:You're not supposed using the side entrance.
Correct:You're not supposed to use the side entrance.
This construction requires supposed to followed by a base verb; supposed + -ing is not grammatical here.
Incorrect:She wasn't suppose to leave without signing out.
Correct:She wasn't supposed to leave without signing out.
Supposed is always spelled with a final -d in this construction, even when it is difficult to hear after wasn't.
Incorrect:The interns wasn't supposed to see the draft yet.
Correct:The interns weren't supposed to see the draft yet.
The verb agrees with the plural subject the interns: use weren't, not wasn't.
Incorrect:It don't supposed to rain this week.
Correct:It isn't supposed to rain this week.
Supposed to in this construction uses be (is/are/was/were), not do-support.
Incorrect:Was you supposed to call the client yesterday?
Correct:Were you supposed to call the client yesterday?
With you in the past, use were rather than was, even when referring to one person.