Question

There is a string $STRING, in which syllables are written with the spaces. If the variable $WORD have at least one syllable in this string, report of this in any way.

Answer 1

Your solution checks to see if $WORD exists in $STRING when it should be the other way around. Try this:

string="run walk stand"
word=walking
if echo "$string" | sed -e 's/ /\n/g' | grep -Fqif - <(echo "$word")
then
    echo "Match!"
fi

As you can see, you can test the result of the grep without having to save the output in a variable.

By the way -n is the same as ! -z.