What do I do if I make a mistake in prayer?

Even the Prophet ﷺ forgot in prayer

Once the Prophet ﷺ prayed an extra rakah by mistake. When the Companions told him, he made two prostrations and said: "I am only a human being; I forget just as you forget. So when I forget, remind me. And if any of you is unsure in his prayer, let him aim for what is correct, complete his prayer on that basis, and then make two prostrations" (Sahih al-Bukhari 401). That is a huge comfort for anyone still learning.

How to do the prostration of forgetfulness

• In the final sitting, after the tashahhud, say "Allahu akbar" and go down into sujood.
• Sit up briefly, then make a second sujood, saying what you normally say in sujood.
• Sit and finish with salam.

Scholars differ on whether it comes before or after the salam. The Hanafis do it after one salam, the Shafi'is before the salam, and the Malikis and Hanbalis before or after depending on the mistake. All of these are valid, so follow what you were taught.

Common mistakes and what to do

• You forgot the first tashahhud (stood up after rakah two): if you're already standing, carry on and do sujud as-sahw at the end. The Prophet ﷺ did exactly this.
• You added something (an extra rakah or bow): do sujud as-sahw.
• You missed a pillar, like a bow or a prostration: go back and do it if you can, or make up that rakah, then sujud as-sahw. If you only remember long after finishing, repeat the prayer.
• You recited the wrong surah or skipped a verse after al-Fatiha: no problem, nothing is needed.
• You stumbled while learning al-Fatiha: do your best; Allah doesn't burden anyone beyond their ability. See How do I memorize short surahs for prayer?

When praying behind an imam

If you make a small mistake behind an imam, you don't do sujud as-sahw; the imam carries it. If the imam makes a mistake, men say "Subhan Allah" to remind him, and he does the prostrations with everyone following. If you joined late, finish your missing rakat after he ends. See How do I pray as a Muslim?