
How about this?
It's a big mistake to assume that you'll learn to speak English just by going to America.

"If you think/thought..., then you're (sorely) mistaken," is a particularly common variation on the "If you..., then (you are wrong/doing something wrong)" phrase. Looking at it again, it probably matches up with the Japanese sentence a bit more than '...big mistake' because '...make a big mistake' could also be taken to mean that the person is enacting or has already enacted some decision based on the incorrect assumption Either way, the grammar pattern alone carries the meaning of an assumption, and flipping it around to "(You are wrong) if you think that..." is acceptable, but less common.
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License: CC BY 2.0 FRLiest van biedroagen
This sentence was initially added as a translation of sentence #3607569
touvougd deur User63086, November 5, 2014
Hinwezen deur User63086, November 5, 2014
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Hinwezen deur Sanaseppo, November 8, 2014