Chinese conditional sentences consist of two clauses: the condition, and the consequence.
Chinese has several expressions related to conditionality, such as "if…then" or "unless". Many of these expressions are translated in English as "if", but the range of Chinese expressions is much larger, and cover more specific situations.
The most common way to say "if" in Chinese is either with 如果 (rú guó), and 要是 (yào shì). These words are equally common and can be used interchangeably.
The other words that can be translated as "if" are as follows. These are not usually used in daily life, and will mostly be found in specialized situations and texts:
若是 – ruò shì
倘使 – tǎng shǐ
倘若 – tǎng ruò
假若 – jiǎ ruò
假如 – jiǎ rú
假使 – jiǎ shǐ
If … Then
Both 如果 (rú guó), and 要是 (yào shi) are usually paired with 就 (jiù), which precedes the complementary phrase of the sentence, as in the following example:
rúguǒ wǎnshang xiàyǔ wǒ jiù bú qù kàn diànyǐng.
If it rains this evening, I won't go to the movies.
Note that the 就 (jiù) may not be translated to English, but is usually necessary in Chinese.
yàoshì wǒ hē kāfēi wǒ huì shuì bù zháo.
If I drink coffee I won't be able to fall asleep.
In order to emphasize the conditional "if", we can use the Chinese word 除非 (chú fēi), which translates as "only if…", "only when…" or "only in the case that…".
为了强调“if”的条件，我们可以使用中文词语“除非”，译为“only if …”、“only when …”或“only in the case that …”。
除非 (chú fēi) is used to stress the outcome of an action or inaction, as in the following examples:
chúfēi yǔ tíng le, wǒ cái huì chūqù.
I won't go out unless it stops raining. (literally: Only if the rain stops, I will be able to go out.)
chúfēi yǒu qián, yào bu rán wǒ shì bú huì mǎi fángzi de.
Only if I were rich would I be able to buy the house.
A related Chinese expression is used to emphasize that something is exactly what is stated.
When used in conjunction with 也 (yě), 就是 (jiù shì) can mean "even if", as in the following example:
nǐ jiùshì zài cōngmíng, bù nǔlì xuéxí, yěbù huì dédào hǎo chéngjī de.
Even if you are intelligent, if you don't study hard, you will not get good marks.
Just as in the examples with 就 (jiù) when combined with 如果 (rú guó), and 要是（yào shì), 也 (yě) when combined with 就是 (jiù shì) is not necessarily translated into English. Nonetheless, it is an important part of Chinese grammar, and your sentences may sound awkward without these helper words.
Translated from: mandarin.about.com
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