Russian negation

Lastly, section 279 of A Comprehensive Russian Grammar states: "Нельзя combines with the imperfective infinitive in the meaning of inadmissibility and the perfective in the meaning of impossibility."

So, your first sentence, "*они пишут по-русски правильно," should read "они правильно пишут по-русски" and your second sentence, "*здесь нельзя говорят громко," should read "здесь нельзя громко говорить."

Today's word:
круг - circle