Advanced Details: Should I write suppose to or supposed to?

You should write supposed to because we usually us this verb phrase with the verb BE to mean expected to or required to.

Be + supposed + to (with the letter -d)

• You are supposed to provide an English translation too.
• We are supposed to modify the equipment.
• They were supposed to leave yesterday. Did they go?
• She was supposed to submit the document this morning, but she is still working on it.
• I am not supposed to tell you this, but …

But we sometimes use “suppose” informally (without the letter -d or the word to) to indicate that you are not sure

• I suppose I had better leave now because the rain has stopped.
• I suppose I could give it a try
• Suppose it’s true, what will you do?
• I suppose it’s too late now?
• Is the picture hanging the right way now? Yes, I suppose so. It looks good to me.