In Qur'anic orthography, some alifs are written but do not follow the ordinary rule illustrated by «أَنَا». Certain words have their own transmitted forms in joining and pausing; in other instances, an extra written alif is not voiced. Spelling alone is therefore insufficient for determining the sound.
Words such as «لَكِنَّا» and «سَلَاسِلَا» are discussed with specific recitational detail. More than one stopping form may be transmitted for a particular word, but such permissions are attached to named forms and readings, not freely extended to all similar spellings.
Correct practice considers the authenticated reading of the word, whether there is a pause or a join, and the signs in the adopted mushaf. Always sounding every final alif, or always deleting it, would both be unsafe generalizations.