Optional components in conditional statements
作為「假使」(if)的句子，如果全部省略假使/假如/如果/要是和就，有的時候會不太容易瞭解，例如下面這個句子就不清楚：

As a IF sentence, if we omit not only 假使/假如/如果/要是 but also 就, sometimes it can become hard to be understood. For example (bad one):


  明天下雨，不去了。(If it rains tomorrow, we will not take the road.)


但是如果加上假使/假如/如果/要是或是就，意思就很明白。也就是說，句型1～6會比較好，而這四個都可以如此構句。

But if we add 假使/假如/如果/要是 or 就, it would sound perfectly clear. In other words, construction 1~6 would be better, and all these four IF-words can be used this way.
這四個假如沒有絕對哪個比較正式，省略的話，也跟正式與否無關。（如果硬要區分，要是比較口語一些）

All these four IF-words are actually not different in terms of formality. And if we omit them, it does not concern the formality, neither. (If we really want to distinguish between them, 要是 is more oral. )