There are many words that are constantly misused like: there, they’re or their. But in order to use them correctly you have to first know what they mean!
Here is a rough guide…
There indicates a place or a situation.
Example: There is so much I have to do before I go on vacation.
They’re is a contraction of the two words: they are.
Example: They’re a wonderful couple and it is such a pleasure to be with them.
Their is a plural possessive: something that belongs to more than one person. If you can substitute more than one name (or thing) and not change the meaning of the sentence, then use their.
Example: Raja and Rani lost their father last week.Similarly a lot of us don’t use the words that and which in the right context.
The rule of thumb would be to use which when the phrase is merely an add-on thought or not essential.
Example: She bought me a lovely vase, which was made of terracotta.
The sentence She bought a lovely vase is complete in itself made of terracotta is not essential in completing the sentence and is just an add-on.
If made of terracotta was essential to the sentence you would use that. Example: Do buy that vase that is made of terracotta.
Now the phrase made of terracotta is needed as it specifies the particular vase to be bought.
Article Source: DECCAN CHRONICLE