What are the acts that require intention and those that do not, and what is the ruling on forgetting the intention in acts that require it, and does abandoning it invalidate a righteous deed, and what is the expiation for that?

Intention (niyyah) is a condition for the validity of purely devotional acts of worship and for receiving reward for all deeds. Forgetting it invalidates purely devotional acts of worship. There are obligatory acts that do not require intention, such as removing impurities, but there is no reward for them without it. Every deed by which a servant seeks to draw closer to Allah will not earn reward unless it is accompanied by the intention of drawing closer. Abandoning the intention results in the invalidation of the act if intention is a condition for its validity, or the non-reception of reward if it is not a condition.