As Sara said ف could mean ( then ) but also ثم means then. One of the diffrences between (ثم) & ف) is, ف faster in the duration of action than ثم. for example: رأى الولدُ الكلب فهربَ... The boy has seen the dog then -directly- he has ran away. رأى الولد الكلب ثم هرب..... it gives you an indication that the boy has seen the dog and after a while he has ran away. 2- ألاّ same as أن لا in the meaning which I think means (not) in English. يجب ألاّ تكذب....you must ((not)) lie قال تعالى (وَلاَ يَجْرِمَنَّكُمْ شَنَآنُ قَوْمٍ عَلَى أَلاَّ تَعْدِلُواْ ).......Allah says in the Quran (and do not let the hatred of a people prevent you from being just.) قال تعالى (وَقَضَىٰ رَبُّكَ أَلَّا تَعْبُدُوا إِلَّا إِيَّاهُ وَبِالْوَالِدَيْنِ إِحْسَانًا ....Allah says (And your Lord has decreed that you ((not)) worship except Him, and to parents, good treatment. )

the mean of ف like in this example is : then قرأت فجلست = I read then I set but it use when you talk about many thing you done the time between first and second is not tall

thank you for all of you :).