Is there certain surahs we must recite in salah? I usually just recite surah Al-Kafirun and surah ikhlas because I do not know any other off by heart.
You can recite anything you wish after al-Fatiha; it can be a whole sura or a verse or two (such as the Throne Verse by itself).
Assalaamu 'Alaikum, I'm confused about what you're allowed to recite after Fatiha during salah. I've been told you need to recite entire surahs after. there's also the fact that the longer the surahs you use the more blessings from God. This is disheartening for me because so far I only know of few short surah since memory isn't my strong suit at all. I yearn to pray tahajjud but I don't know enough to surahs to recite off the top of my head. I feel my prayers hold less value to God due to this.
Alaikumassalam wa rahmatullah,
You can recite anything you wish. In the Shāfiʿī, Ḥanbalī and Mālikī schools whatever you recite after al-Fātiḥa is considered voluntary rather than obligatory (so you can simply recite al-Fātiḥa). The Ḥanafī school considers it wājib, which is a degree lower than farḍ (obligatory) but close to it.
It is recommended that you recite a full sura. But reciting a single verse is also permissible. Imam Aḥmad (founder of the Ḥanbalī school) recommends that if you recite a few verses then they should be long ones.
It is also permissible to recite multiple suras after al-Fātiḥa if you feel like it. This is what some of the Companions used to do.
One way to pray tahajjud when you have not memorized much Quran is to perform two units, then take a break to read a few pages of Quran, then get up to pray some more.
You can also hold a book of Quran in your hand and read from it during prayer. Aḥmad says there is no issue with doing this in voluntary prayers, while al-Zuhrī,(an important early Medinan scholar) says “the best of us used to do that” (to read from a book of Quran during voluntary night prayers in Ramadan).
Sources for these opinions (all in Arabic): Fatwa 1 | Fatwa 2 | Fatwa 3 | Fatwa 4