Personal language, including the words people use and other characteristics of how they speak or write, is called their idiolect. Idiolect is similar to dialect; however, dialect relates to the way a group of people speak or write.
An idiom is a phrase that is easily understood by the speakers of a particular language; however, the meaning is different from the normal meaning of the words.
Example: A bird in the hand is worth two in the bush.
Example: a chip on your shoulder
Example: a drop in the bucket
If is often used in casual speaking. If is used when there is a condition.
Incorrect: If you don’t know if [should be whether] the front door is locked, you better get up and ...