Question

How do I know when to use a masculine or feminine word?

Answer 1

Most masculine words words end with
anything other than a, except when there's ma and pa at the end.

And most feminine words end with
a, sion & ad

but there are expetions:

http://www.studyspanish.com/lessons/genoun2.htm

Answer 2

masculine words end in "o" most oof the time, while fem end in "a" use un, el, and los before masc. and una, la, and las before fem