Theory:

Sentences in English are often made up of CLAUSES which are joined together with connecting words like AND, BUT or BECAUSE.
A CLAUSE is the smallest group of words that make sense.

AND can be used in many ways, but the most common is to connect clauses that have the same idea.
Example:
David is clever AND he is very good at sports.

BUT is often used when a clause is in contrast to another clause.
Example:
David is clever BUT he is not very good at sports.

BUT is not normally used at the beginning of a sentence.
In such a case, HOWEVER would be used, followed always by a comma.
Example:
David is clever. HOWEVER, he is not very good at sports.

If a clause has a WHY - BECAUSE relationship with another clause, BECAUSE is usually used.
Example:
He ate the sandwich - He was hungry.
WHY did he eat the sandwich?
He ate the sandwich BECAUSE he was hungry.

Sometimes one clause is used to give more information about another clause. This is called a RELATIVE CLAUSE.
(There is a separate support document on RELATIVE CLAUSES.)

The first words in clauses like these are words like WHO, WHERE, WHICH or WHEN, and these words are called RELATIVE PRONOUNS.
Example:
This is the man WHO lives near my house.(WHO refers to man, a person.)
This is the shop WHERE I buy my clothes.(WHERE refers to shop, a place.)
This is the car WHICH I bought yesterday.(WHICH refers to car, a thing.)
This is the day WHEN I get married.(WHEN refers to time.)

Sometimes WHO, WHICH and WHEN are replaced by THAT, but THAT cannot replace WHERE.

The word IN is a preposition and is used to sentences to show when something is completely surrounded by a place.
Example:
IN the airport, means completely surrounded by the airport.
IN is also used for time, like IN 1200.

The word AT is also a preposition that is used in sentences to show position, and usually means THERE but NOT SURROUNDED BY.
Example:
He is at AT the bus stop.(next to the bus stop.)

AT is also used for time.
Example:
I will see her AT 6:00.

If COMPARATIVE ADJECTIVES like BIGGER and SUPERLATIVE ADJECTIVES like BIGGEST are used in a sentence, THAN and THE must always be used to give BIGGER THAN or THE BIGGEST.
Example:
He is TALLER THAN her.
She is THE SMALLEST.