假设(jiǎ shè) - to assume, to suppose; hypothesis, assumption in Mandarin

假设 (jiǎ shè) works as both verb and noun. As a verb it opens a conditional you know is not true, or not yet true: 假设你是我 supposing you were me. As a noun it is the hypothesis or assumption itself, the thing a plan or an argument rests on.

The register is bookish. In science writing, maths and business analysis it is everywhere; in ordinary conversation the same job goes to 如果 or 要是, and 假如 sits in between. Saying 假设你饿了我就做饭 to a flatmate sounds like you are reading from a textbook.

The trap is 假 itself. Here it is jiǎ, third tone, meaning false or provisional. The identical character read jià, fourth tone, means holiday: 放假 is to break up for the holidays. Get the tone wrong and 假设 stops meaning anything. The other trap is English "assume" in the everyday sense of I assumed you knew, which is 以为, not 假设. 假设 is the deliberate assumption of a reasoner, never a mistaken belief.