Apostrophe usage for pinyin
I noticed some words use an apostrophe such as Xi'ai (喜爱) and Xi'an (西安). I Cannot discern which tonal pronunciation should be used. Why is this?

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the apostrophe is not for tone but the separation of words.  refer to this word lǐxiǎng, those small V are the marks for tone.  do I help?

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xi'an(西安) xi'ai（喜爱）

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if you wanna type 喜爱 you spell xi ai   , 西安 you need xi'an    notice use ( ' )

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The apostrophe is used before the vowels to seperate them from the syllable before
For example：Xi‘an（西安）-----Xian（线，先，县.......）