documentclass{article}

begin{document}

[sum_{x=3}x=3]

[sum_{displaystyle x=4}x=4]

[sum_{x,=,5}x=5]

{thickmuskip=22mu
[sum_{displaystyle x=6}x=6]
}

fontdimen6scriptfont2=1in
fontdimen6textfont2=1in

[sum_{x=7}x=7]


[sum_{textstyle x=8}x=8]


end{document}

begin{align*}
f(x) =& x^2! +3x! +2 \
f(x) =& x^2+3x+2 \
f(x) =& x^2, +3x, +2 \
f(x) =& x^2: +3x: +2 \
f(x) =& x^2; +3x; +2 \
f(x) =& x^2 +3x +2 \
f(x) =& x^2quad +3xquad +2 \
f(x) =& x^2qquad +3xqquad +2
end{align*}

sum_{x = 4}