Learn Chinese Grammar: 要是(yàoshi=if) vs.如果(rúguǒ=if):

要是 and 如果 are often inter changeable. 如果 is more formal and more commonly used in written language. 要是 is more colloquial and more commonly used when expressing hypothetical situations.

Examples:

1． If anyone calls on the phone, tell them I have gone to the client's company for attending the meeting.

如果(rúguǒ=if)有(yǒu=there is)人(rén=person)打电话(dǎ diànhuà=calls on the phone)，就(jiù=then)说(shuō=to tell)我(wǒ=I)到(dào=to go) 客户(kè​hù=client)公司(gōngsī=company)开会(kāihuì=to attend a meeting)了(le)。 (More formal and more commonly used in written language)

2. If I can fly, I will come back often.

要是(yàoshì=if)我(wǒ=I)会(huì=will)飞(fēi=to fly), 我(wǒ=I)会(huì=will)常常(chángcháng=often)回来(huílái)。(More colloquial and more commonly used in hypothetical situations)