Is this phrase suitable for a thing or for a secret? It means "I will keep this secret to myself" = I will not tell anyone, or "I will keep this thing for myself"? Can a native speaker explain?
As a German native speaker I understand the German sentence "Ich werde es für mich behalten." as referring to a secret or other kind of information I promise not to tell anyone.
It took a while until I had an idea how I would say "I will keep this thing for myself" in German: "Ich werde es für mich selbst/selber/persönlich behalten/aufbewahren." With "selbst" , "selber" or "persönlich" it is not suitable for a secret. You can also say: "Ich werde es selbst/selber benutzen." - "I will use it myself." or "Ich nehme es selbst/selber"-"I take is myself." or "Ich nehme es für mich selbst/selber." - "I take it for myself."
P.S.: Before you ask: "selbst" is a bit more formal and "selber" is a bit more informal. Both mean the same.
I guess that in right context it can have both meanings. Check example number five, it has similar construction like Duo sentence...
So how would you say, "I will keep it for myself", which is what I thought this sentence meant?
This is an incorrect translation. The operative phrase is to keep something to "oneself", so the translation should be "I will keep it to myself."
In the translation it says "to myself", doesn't that indicate the dative so "mich" would be "mir"?