'They're sitting in their car there." Simple, yes? Yet we constantly confuse these three words, confounding our listeners and readers mightily. Mastery of this trio of words will make you an excellent English scholar, I guarantee you. Let's look at the definition of these words:
Briefly, "they're" is "they are" contracted. "Their" and "theirs" are possessive forms for "they" - "Their food is theirs alone." "There" means "in that place," and you can even imagine a finger pointing out the spot. "There is the opposite of here." Then we have "there's" standing for "there is." Yes, English is a complicated business indeed.
If English is your second language, it will pay you to practice lessons aloud so that your ears can grow accustomed to the correct sounds. Our ears need training every bit as much as every muscle in the body. Once the correct usage of words become comfortable, it will be easy to choose the correct pronouns.
When, you ask, do we use "there is" instead of "there are"? Little kids say "there is trucks" instead of "there are." "There's" is too easy to say instead of "there are," so you have to govern yourself carefully to use the correct version. The two "r's" in "there are" throw us easily; beware that pesky contraction, especially when you are speaking.
Another problem arises with "there," and we should clean it up now before it becomes downright cantankerous. Most languages have a construction called "existential," meaning these sentences are used for pointing out the existence of something. In English the adverb "there" plus a linking verb (usually a form of "be") serves this purpose. The problem arises when choosing the verb to go with the noun. Too often we see:
"Madam, there are no need to cry." (Wrong)
"Madam, there is no need to cry." (Correct)
There is only one answer to that problem.
President Obama, there is a problem.
Note that in the above sentences, the subject agrees with the verb. "Need - is," "Parts - are." " Problem - is." If I changed the subjects to plural, they would read thusly:
"Madam, there are several reasons to cry. (reasons are)
There are several doses left in that medicine bottle. (doses are)
President Obama, there are several problems. (problems are)
Now you try:
1 - There (are/is) dozens of eggs spilled on the carpet.
2 - Minnie Mouse and Mickey Mouse chased (her/their) tails.
3 - Is the flag (their/there)?
4 - (They're/their) going away.
5 - Why am I going (their/they're/there)?
The writer is an author, teacher and body language expert.