What is the ruling on memorizing the Qur’an then forgetting it? Is it obligatory to memorize the Qur’an or to learn its Tajweed? Will a person be held accountable for what they have memorized of the Qur’an?

Memorizing the Quran is a great blessing and a high rank. The Prophet, peace and blessings be upon him, encouraged reviewing and maintaining it, saying: "Guard the Quran, for by Him in whose hand is Muhammad’s soul, it is more liable to escape than a camel from its hobble." The early Muslims used to preserve what they had memorized and disliked forgetting the Quran. It is incumbent upon a Muslim to memorize what is necessary for prayer, such as Al-Fatiha and a [short] surah, and it is proper to learn its tajweed.