Formal:
* Não saber falar: This is the most literal translation and is suitable for formal situations.
* Não dominar a língua: This means "not to master the language" and is more formal than "não saber falar".
Informal:
* Não saber falar: This is also commonly used in informal situations.
* Não entender nada: This means "not understand anything" and is more informal than "não saber falar".
* Não falar nada: This means "not speak anything" and is even more informal than "não entender nada".
Example sentences:
* Formal: "Eu não sei falar português." (I don't know how to speak Portuguese.)
* Informal: "Eu não entendo nada de português." (I don't understand anything in Portuguese.)
You can also use the verb "falar" with the negative pronoun "não" followed by an infinitive verb:
* Formal: "Eu não posso falar português." (I can't speak Portuguese.)
* Informal: "Eu não consigo falar português." (I can't manage to speak Portuguese.)
Remember that the best way to say "not know how to speak" in Portuguese will depend on the situation and the relationship between the speakers.