Is the intention accepted after completing a righteous deed for which the intention was forgotten before starting it, and how can one accustom himself to forming the intention before every action?

The fundamental principle concerning intention (niyyah) is that it must precede the act of worship or be simultaneous with it, never after. Thus, the intention for Zakat is not valid after the money has been paid, nor is the intention for a fard (obligatory) prayer valid after one has begun it, nor is the intention to remove ritual impurity valid after performing ghusl (ritual bath). This applies to obligatory or recommended acts of worship that are distinguished only by intention. As for other acts of righteousness, some scholars hold that a Muslim is rewarded for them even without a specific intention, considering a general good intention to be sufficient.