I saw this in an instant message.

Problem:
The verb “suppose” is incorrect here.

Explanation:
The statement is written in abbreviated passive voice, albeit incorrectly. The subject of the sentence (Jon) is acted on by a verb, and we do not know who is supposing that Jon will “present it”. A statement in active voice must identify the person who is supposing that Jon will “present it”.

“Jane supposes that Jon will present it.” is an example of a statement in active voice. We get “Jon is supposed by Jane to present it.” when we rearrange the statement to put it into passive voice. Abbreviating this statement, we correctly get “Jon is supposed to present it.” in abbreviated passive voice.

In other words, “supposed” — not “suppose” — is the passive form of the verb “to suppose”.

I believe that the use of “suppose” instead of “supposed” when the latter is appropriate represents

• ignorance about passive voice, and
• a laziness that favors more easily pronounced words.

The word “suppose” is easier than the word “supposed” to say. Throw in a lack of understanding how sentences in the active voice are converted to sentences in the passive voice or abbreviated passive voice, and it’s no wonder that some English speakers choose “suppose” (incorrectly) instead of “supposed” in a statement made in the passive voice or abbreviated passive voice.

Solution:
“Jon is supposed to present it.”