The tones in Chinese Hanyu pinyin can change the way the word sounds with tone marks. For example, third tone placed over the O in Wǒ makes the translation me, i. but if the fourth tone is placed over the O, The word becomes Wò, which is a mark of scolding.
Where to place the tones
The tone in a word is placed over the final on which your mouth opens the greatest. For example, in the word Zhōng the tone is placed over the o because it comes before any of the other letters. The tone always goes over A; if no a is available then the tone is placed over the O, if no O, Over E, if no e placed over I, if no I placed over U if no U is placed over ü. The only exception to this rule is when the final I is followed by U, then the tone is placed over U.