For strength:
* Débil: This is the most common and general opposite of "fuerte" meaning weak.
* Frágil: This means fragile or delicate.
* Incapaz: This means incapable or unable.
For loud:
* Suave: This means soft or gentle.
* Bajo: This means low or quiet.
* Silencioso: This means silent.
For strong:
* Débil: This can also be used for "strong" in a figurative sense, meaning weak or powerless.
* Flojo: This means loose or slack.
* Insignificante: This means insignificant or unimportant.
For firm:
* Blando: This means soft or yielding.
* Flexible: This means flexible or adaptable.
* Débil: This can also be used for "firm" in a figurative sense, meaning weak or easily influenced.
For hard:
* Blando: This means soft.
* Suave: This means smooth or gentle.
* Flexible: This means flexible or adaptable.
To give you a more precise opposite, please provide the context in which you are using the word "fuerte."