Mandarin Chinese is extremely consistent with grammar. There are (compared to English) relatively few grammatical “exceptions”.
When it comes to pronunciation, however, there are a couple of strange transformations that occur with some “special characters”. These characters are referred to in Mandarin as “dūo yīn zì” “多音字” ＝directly translated as “many toned characters”. They are exactly what they are translated to be. In some circumstances these “special characters” change tonally (i.e. a character that is normally a first tone becomes fourth etc.) while in others, like 行 or 了, they are completely new words pronounced completely different.
(Hopefully, on a later date either myself or other Crazina members can compile a list of these special characters, but as for now I am just going to talk of two of the most commonly used 多音字.)The first one is actually “one” or “一” . Most commonly ( including most dictionaries) “一” is said to be pronounced “yī ” with a first tone. However, that is not all this little one stroke character is capable of doing!!! “一” is somewhat of a super-mutating “special character” with many different forms. 
Here they are:
1. yī = only when ” 一 ” is used alone, or when it is at the end of a word or sentence is it pronounced with a first tone. (i.e. shíyī “十一 ” or yī yī dé yī “一一得一”.
2. yí = when used before a character with a fourth tone (falling tone) it is pronounced with a second tone. (i.e. yíbàn “一半”, yígòng “一共”)
3. yì = when used before a character with a first, second or third tone it is pronounced with a fourth tone. (i.e. yìtīan “一天”, yìnían “一年”, yìdiǎn “一点” .
NOTE: Don’t let these different rules get you down take them one at a time and practice the rule over and over, and in no time unconsciously you will be saying all of them correctly.
The next special character “不” (not, no) Recognized by most, and dictionaries, as bù (fourth tone). This special character, just like it’s friend “一“, also has a couple of tricks up it’s sleeve (when I say a couple I mean ONE!). Enjoy!!!
1. bú = Only when “不 ” is in front of another character with a fourth (falling) tone does it change into a second tone. All other instances it will remain a fourth tone. i.e. búhùi “不会”, búshì “不是 “.
2. yǒu “有”= “to have” “不” is never to be used with yǒu “有”!!! You MUST say měi yǒu “没有” to say “not have” It is super nasty sounding, and very incorrect, to say “bù yǒu” “不有”.
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