A statement is an assertion that can be determined to be true or false.
The truth value of a statement is T if it is true and F if it is false.
For example, the statement ``2 + 3 = 5'' has truth value T. Statements
that involve one or more of the connectives ``and'', ``or'', ``not'',
``if 
then'' and `` if and only if '' are compound
statements (otherwise they are simple statements). For example, ``It is
not the case that 2 + 3 = 5'' is the negation of the statement above. Of
course, it is stated more simply as ``2 + 3 
5''. Other examples of
compound statements are:
If you finish your homework then you can watch T.V.
This is a question if and only if this is an answer.
I have read this and I understand the concept.
In symbolic logic,
we often use letters, such as p, q and r to represent statements
and the following symbols to represent the connectives.
Note that the connective ``or'' in logic is used in the inclusive sense (not the exclusive sense as in English). Thus, the logical statement ``It is raining or the sun is shining '' means it is raining, or the sun is shining or it is raining and the sun is shining.
If p is the statement ``The wall is red'' and q is the statement
``The lamp is on'', then 
is the statement ``The wall is red or
the lamp is on (or both)'' whereas 
is the statement
``If the lamp is on then the wall is red''. The statement 
translates to ``The wall isn't red and the lamp is on''.
Statements given symbolically have easy translations into English but it
should be noted that there are several ways to write a statement in
English. For example, with the examples above, the statement
directly translates as ``If the wall is red then the lamp
is on''. It can also be stated as ``The wall is red only if the lamp is on''
or ``The lamp is on if the wall is red''. Similarly, 
directly translates as ``The wall is red and the lamp is not on'' but it
would be preferable to say ``The wall is red but the the lamp is off''.
Click on the microscope
for a more extensive list of English equivalents.
The truth value of a compound statement is determined from the truth values
of its simple components under certain rules. For example, if p is a
true statement then the truth value of 
is F. Similarly, if
p has truth value F, then the statement has truth value T.
These rules are summarized in the following truth table.
If p and q are statements, then the truth value of the statement
is T except when both p and q have truth value F. The
truth value of 
is F except if both p and q are true. These
and the truth values for the other connectives appear in the truth
tables below.
From these elementary truth tables, we can determine the truth value of
more complicated statments. For example, what is the truth value of
given that p and q are true? In this case, 
has truth value F and from the second line of the tables above, we see the
truth value of the compound statement is F.
Had it been the case that p was false and q true, then
again would be false and from the fourth row of the above table
we see that is a false statement.
To consider all the possible
truth values, we construct a truth table.
The lower case t and f were used to record truth values in intermediate
steps.
Note that while a truth table involving statements p and q has 4 rows
to cover the possibility of each statement being true or false, if we have
additional information about either statement this will reduce the number
of rows in the truth table. If, for example, the statement p is known
to be true, then in constructing the truth table of 
we will only have 2 rows. Truth tables involving n statements will have
rows unless additional information about the truth values of some of
these statements is known.
A statement that is always true is called logically true or a tautology. A statement that is always false is called logically false or a contradiction. Symbolically, we denote a tautology by 1 and a contradiction by 0.