أنا لا أَكتُبُ إلّا لُغَةً
في فُؤادي سَكَنَت مُنذُ الصِّغَر
— صباح حكيم
Notice that the word لغة here is in the accusative case. Why is this?
The accusative case in Arabic is called منصوب. This literally translates to “something that has been pitched”, such as a tent pitched to the ground. In other words, the accusative is something that has been added to a sentence. The sentence is grammatically complete without it, but it gives additional information.
The sentence أنا لا أكتب is grammatically complete, but more information is required to give the intended meaning. You “pitch” the additional information onto the sentence, and therefore the additional information is منصوب, i.e. in the accusative case.
The word إلّا is called أداة استثناء (a tool of exception). It is added onto a sentence to provide an exception. The sentence translates to, “I do not write except a language…” And because the exception is an additional piece of information, the word that comes after the tool of exception is in the accusative case.