When speaking to a female.

When speaking to a female.
 

Hi. Sorry about my question but I need to have clarification. This is something I should have asked when I was a beginner. In some words in Spanish when I speak to a female, the 'o' changes to 'a' - how do I know what words change? For example?

Amigo/amiga
Bonito/bonita
Fantástico/fantastica
Estás seguro/estas segura.

Thanks.

All adjectives ending in "o" change to "a" when you're referring to a woman. I don't remember whether there are exceptions. In case of nouns, some suffer changes.

Gracias por la clarificaron. :)