如果 is the standard, neutral conjunction for 'if' in Chinese, used for general conditions. It is more formal than the colloquial 要是 (yào shi) and does not carry the 'just in case' nuance of 万一 (wàn yī) or the 'unless' meaning of 除非 (chú fēi). Use 如果 for most if-clauses in both writing and speech.