Cfrgtkky

i would like to put equations below each other and have equivalences next to it below each other as well.
Usually i use the command &= to put equations below each other, but when i use that it completely misunderstand the syntax.
Another problem is now that i have a line which is to long to fit in one row so i want to put the addition-symbol below each other and still want the equation below each. I tried:

documentclass[10pt,a4paper]{article}

usepackage[utf8]{inputenc}

usepackage{amsmath}

usepackage{amsfonts}

usepackage{amssymb}

begin{document}

begin{align*}

P(X(t+h)=n)&=P(bigcuplimits_{k=0}^n{X(t)=k, X(t+h)=n})\

&=P(X(t)=n)cdot P(X(t+h)=n | X(t)=n)\

&+P(X(t)=n-1)cdot P(X(t+h)=n | X(t)=n+1) \

&=....

end{align*}

end{document}

But whenever i want to put another & somewhere to have the mentioned term below it, it completely destroys my whole syntax.

Second Problem:

Leftrightarrow a&=b \

Leftrightarrow abcdefgh&=b \

Here I have the equations below each other but i want the equivalences to be below each other as well at the left side or wherever i want to put them.

edit:
your question and additional request in comments are not clear ... now i added two more possibilities to arrange of your equations:

documentclass[10pt,a4paper]{article}

usepackage[utf8]{inputenc}

usepackage{mathtools, amssymb}



begin{document}

begin{align*}

P(X(t+h)=n) & = P(bigcuplimits_{k=0}^n{X(t)=k, X(t+h)=n})\

            & begin{multlined}[t][0.7linewidth]

              = P(X(t)=n)cdot P(X(t+h)=n | X(t)=n)\

                        +P(X(t)=n-1)cdot P(X(t+h)=n | X(t)=n+1)

                end{multlined}     \

            & = dotsm

end{align*}

or

begin{align*}

P(X(t+h)=n) & = P(bigcuplimits_{k=0}^n{X(t)=k, X(t+h)=n})\

            & = P(X(t)=n)cdot P(X(t+h)=n | X(t)=n)\

            &qquad +P(X(t)=n-1)cdot P(X(t+h)=n | X(t)=n+1) \

            & = dotsm

end{align*}

and

begin{alignat*}{3}

Leftrightarrow &;& a        & = b \

Leftrightarrow &  & abcdefgh & = b 

end{alignat*}

or 

[setlengtharraycolsep{2pt}

begin{array}{ccl}

Leftrightarrow & a        & = b \

Leftrightarrow & abcdefgh & = b

end{array}

]

end{document}