Learn Portuguese (Portugal): Mas não lhe dês o osso.

In Portuguese, negative words like não act like magnets that pull object pronouns before the verb. In an affirmative command, you would attach the pronoun to the end of the verb with a hyphen (like dá-lhe), but the negative word não forces it to come before the verb.

Why is the verb dês and not dá?

When you give a negative command (telling someone not to do something), you cannot use the normal imperative form. Instead, you must use the present subjunctive form of the verb. If you were telling someone to give the bone, you would use the regular affirmative imperative dá. But because it is a negative command, it changes to não dês.

Could I write não dês-lhe instead?

No, that is a common trap! Because the sentence is negative, the não strictly requires the pronoun to come before the verb. You can never use a hyphen to attach a pronoun to the end of a verb in a negative clause in Portuguese.

Why do we use lhe instead of o to mean him?

We use lhe because it is an indirect object, which answers the question "to whom?". You are not giving "him" away; you are giving the bone "to him". The bone is the direct object (o osso), and the receiver of the bone is the indirect object (lhe).