假使 + [clause],(就 / 那么) + [clause]
[subject] + 假使 + [verb phrase],[clause]
假使 means "supposing" or "if". It belongs to writing and careful speech, and it asks the reader to imagine a situation. The result clause takes 就, 便 or 那么.
假使 is at home with conditions that are false or unlikely – "if I were", "had it not". The result clause then often carries 就不会 or 早就.
In essays and debate, 假使 sets up a premise and 那么 draws the consequence.
假设 is a verb and a noun – "to assume", "a hypothesis" – as well as a conjunction. 假使 is only a conjunction, so it can't be the object of a verb or be the main verb itself.
倘若 and 假如 can replace 假使 in most sentences; each has its own point.