如果 alone opens the supposition; 就 is the optional pickup that fires the consequence. The frame names the whole condition→consequence pairing; this entry is the opening marker.
treating it as a fact: 因為明天有空,如果我們去 → 如果明天有空,我們就去
marker on the wrong clause: 下雨如果,我就不出門 → 如果下雨,我就不出門
就 before the subject: 如果有空,就我去 → 如果我有空,就去 / 我就去
English 'if' is one word at the clause front; learners stop there and drop the 就/那 pickup the consequence wants, or confuse 如果 with the factual 因為.