While suppose and supposed are two different terms based on their functions, many people still find it confusing to decide when to use one over the other. This may be understandable as these terms are two different forms of the word suppose.
Suppose is a word used as a verb meaning “to assume that something is the case on the basis of evidence or probability but without proof or certain knowledge.”
“NELSON: No good deed goes unpunished, I suppose”
“A Jewish ‘Christmas’: ‘I suppose my mother had explained the cross-cultural situation to Santa Claus'”
“Letter: Suppose Trump could eliminate the CIA”
New Bern Sun Journal
It may also be used as a verb meaning “to be required to do something because of the position one is in or an agreement one has made.”
“It was supposed to be a marriage made in heaven, but Ronald Koeman’s Everton reign could end in divorce after just six months”
“‘Is that supposed to hurt my feelings?’: Patriots shrug off what Mike Tomlin said about them”
The Washington Post
“How am I supposed to find the perfect Christmas ‘party look’?”
On the other hand, supposed is a term used as an adjective meaning “generally assumed or believed to be the case, but not necessarily so.”
“‘A Disgrace … Nonsense’: Trump, Allies Blast Reports On Supposed Info Held By Russia”
“Trump calls supposed delay in hacking intel briefing ‘very strange'”
“What Barcelona think about Man City’s supposed offer for Rakitic”
One way to remember which term you should use in a sentence is to use the following mnemonic. If you want to use the term as a verb, then you should choose suppose since both have an “e” in their spelling. Meanwhile, you should use supposed if you want an adjective as both words have a “d” in their spelling.