In Spanish if you want say: " you could say...". You should say: "Tu podrías decir.../ Podrías decir..." and if you want say " you couldn't say..." would be something like: " tu no podrias decir.../ no podrias decir...".For a question, " could you help me?" would be something like:"¿Podrías ayudarme?".Some examples:
I could do my homework= yo podría hacer mi tarea
you could play soccer= Tu podrías jugar fútbol
Could you help me with English please? = ¿Podrias ayudarme con el ingles por favor?/¿Me podrias ayudar con el ingles por favor?
In Spanish you can also, do a question with in negative form. For example: " ¿No podrias ayudarme con mi tarea?"I'm not sure, but i think that this isn't common in English.