German!! WHAT IS THE DIFFERENCE BETWEEN EIN, EINE, EINEN?

GAAAAAAH!! so frustrating!
like my german friend says it has something to do with singular and plural s and no s
but than theres femine things and male thing!!! D:
i cant tell which is femine or not!!! like Stadt. Das is eine schone Stadt..
female? DX
im going to explode.
..plz help.
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