"my assumption is that" is correct and is commonly used in written English. This phrase is typically used to introduce information that someone believes to be true, but has not been established as true. For example, "My assumption is that the new vaccine will be available in the next few weeks."
"But my assumption is that will come," he said.
"My assumption is that he is still alive".
My assumption is that it's people who will never farm".
"My assumption is that he wanted to go where he's going".
My assumption is that if people don't like the first line they're not reading any farther.
"But my assumption is that there was radiation there," he said.
My assumption is that they will turn out to be conservative".
My assumption is that the handicapped spot is (geographically) the best option, but not fundamentally different from the regular spots.
My assumption is that you smoke copious amounts of weed, and that is why you laughed through the entire apology.
"My assumption is that Celtic, amongst other clubs, are looking at potential candidates.
While this isn't one of them, my assumption is that Supreme Court review is a foregone conclusion.
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Ha Thuy Vy
MA of Applied Linguistic, Maquarie University, Australia