keep
The German word 'behalten' is used to express the idea of keeping something, retaining it, or holding onto it. It can be used in various contexts, such as 'Ich behalte das Buch' (I keep the book), 'Er behielt die Erinnerung an sie' (He kept the memory of her), or 'Sie behielt ihre Meinung bei' (She stuck to her opinion).
I should keep that in mind.
The phrase 'Ich sollte das im Kopf behalten.' is used when you want to remember something important. 'Behalten' in this context means 'to keep' or 'retain'.
Can you keep this secret to yourself?
The phrase 'Kannst du dieses Geheimnis für dich behalten?' is asked when you want the listener to not reveal a secret you are sharing. 'Behalten' is used to imply maintaining the current state of knowledge without sharing it.
They want to keep the money.
In the phrase 'Sie möchten das Geld behalten.' the usage of 'behlaten' means to retain possession, in this case, of the money. It suggests that they don't wish to spend or give away the money.