The structure 要是…，否则… is used in compound sentences that consist of three parts.
In the first part of the sentence, we should state the condition as: "if this situation happens...". This part should begin with the adverb 要是, and we should state the subject right after it.
In the second part of the sentence, we should state the first possible result as: "...then it will happen".
We usually also state the word 就 before the predicate.
In the third part of the sentence, we name the second possible result: "...otherwise it may be so". This clause should begin with the conjunction 否则.
possible result 2
If he does not go, I won’t go either. It will be too dangerous for me to go alone
yàoshi tā bú qù, wǒ yě bú qùle, fǒuzé wǒ yí ge rén qù tài wéixiǎn le
If it rains tomorrow, we won't go hiking. Otherwise we'll catch a cold
yàoshi míngtiān xiàyǔ, wǒmen jiù bú qù páshān le, fǒuzé huì dòng gǎnmào de
If you don't bring a bag, I'll bring one. Otherwise we don’t have anything to carry our purchases in
yàoshi nǐ bú dài bāo, wǒ jiù dài yí ge, fǒuzé mǎi de dōngxi méi dìfang fàng