How should elements in a matrix be represented using variables?

[#1]

How should elements in a matrix be represented using variables? For example, in following JSL, [1 -3] works properly, but [1 a] causes an error. How can this be fixed?

Names Default To Here( 1 );
Word( [1 -3], "This is an apple", " " );
a=-3;

Word( [1 a], "This is an apple", " " );

[#2]

Depending what you are trying to do, you can build it from a list for example

Names Default To Here( 1 );

a =-3;

l = Eval List({1, a});

Word(Matrix(l)`, "This is an apple", " " );