"I forgot his birthday!"
It an answer with 忘记 instead of just 忘 also acceptable? I got marked wrong for using 忘记.
How do you know when to use each version. Are there certain criteria that dictate which one to use.
It is essentially a combination of what is commonly used, what sounds smoother （比较顺）, what the context is and so on, hence the best way to know is through experience e.g. listening, reading, speaking, and practice in and usage of the language in general.