If we say that someone can write the meaning can be interpreted in two different ways. It is possible that he learned to write at school and that in general he is able to do it, however that doesn't mean that in a concrete moment he can do it. If he has his two arms broken, he can't write, although he learned it. The meaning can be as well that at a certain moment there is no hindrance and that he can actually write. In Portuguese there are two completely different verbs for that. If someone sabe, knows, to write, he has at least learned it. If he pode, can, write, he is actually able to do it in a concrete moment. To keep it short and simple. A sentence like "Ele não sabe dormir" doesn't make any sense. If we say that we suppose that a learning process is needed in order to perform this action, to sleep, what is not the cases. The only way to say that is "Ele não pode dormir". Although both sentences are translated the same way, there is only one correct translation. This distinction gets lost if we translate in English.
1) Não pode ler.
He can't write.
2) Não sabe ler.
In 1) he can't read because he lost for example his glasses. 2) means that he never learned it. His incapacity is not due to certain circumstances.
Choose the correct option in the following sentences. The meanig can be deduced from the context or from the English translation. The difference betweeen conseguir and poder is the same as in English between to manage and can. To manage describes a long lasting effort. The meaning of "He managed do thata" and "He can do it" is not the same.