Where the tanween alif goes

An indefinite noun in the accusative ends in an extra alif: رأيت ولدا, اشتريت كتابا, شكرا. It marks direct objects, predicates of كان, and many adverbs such as أيضا, جدا and دائما.

There is no extra alif after taa marbuta (رأيت سيارة), after an alif and a hamza (شربت ماء, رأيت سماء صافية), or after an alif maqsura (رأيت فتى).

A definite noun never takes it, with ال or as the first term of an إضافة: رأيت الولد, قرأت كتاب أخي.