See my correction below, including questions [in square brackets]. Please note that I am not sure about all of these corrections, as I am not familiar with the topic you are talking about. However, I am happy to discuss this with you (in English :)) over Skype, if you like. 

For these methods, boundary condition formulation is a problem. Because the wall's [is singular okay, or should it be plural 'walls''?] conductivity is finite, boundary conditions on a perfectly conducting plane are not to be implemented ['to be implemented' suggests the SUBJECT (that is, the person implementing) is very important to this idea. Is that true? Or, could you say 'cannot be implemented'?]. Other authors have obtained some boundary conditions [This sentence is very vague. Is it possible to be more specific? If not, that is okay]. The first was Leontovich's boundary conditions [is plural okay, or should this be singular?], but there are more sophisticated expressions that bound tangential parts of electric and magnetic fields.