I saw a sentence in a German textbook:
Aber wenn ich Geburtstag habe, dann darf ich mir ein Essen aussuchen.
I would like to ask why we should use "mir" here? I saw it somewhere else a sentence
Ich darf mich vorstellen
Here "mich" is used instead of "mir". Can someone please explain the differences?
You can translate »mir« as »to me« (in the meaning of »for me«):
Ich suche Essen aus.
I choose food.
The food can be dedicated to me, but also to my neighbor, to my boss, or just anyone. This sentence just says, that i make a decision, but it doesn't say, who is the beneficiary of this decision.
Now with »mir«:
Ich suche mir Essen aus.
I choose food for me.
Now it is clear: The food is dedicated to me. I am the beneficiary of this decision.
You also could say:
Ich suche dir Essen aus.
I choose food for you.Ich suche meiner Tochter Essen aus.
I choose food for my daughter.
What is said above has nothing to do with:
Ich stelle mich vor.
I introduce myself.Ich stelle dich (meinem Chef) vor.
I introduce you (to my boss).
Also compare:
Walter(nominative) stellt mir(dative) seine Frau(accusative) vor.
Walter introduces his wife to me.Walter(nominative) stellt mich(accusative) seiner Frau(dative) vor.
Walter introduces me to his wife.
Here you tell in dative case who is the person that is introduced (the object of introduction). And in accusative case you tell to whom this person is introduced (the target of the introduction). In nominative case you will find Walter, he is the person who performs the introduction.
In german you define the roles not by position but by grammatical case, which gives German a much greater flexibility in word order than English. So also this sentences are correct sentences (some are unusual, but still correct):
Mir stellt Walter seine Frau vor.
Seine Frau stellt mir Walter vor.
Seine Frau stellt Walter mir vor.
Walter introduces his wife to me.Mich stellt Walter seiner Frau vor.
Seiner Frau stellt mich Walter vor.
Seiner Frau stellt Walter mich vor.
Walter introduces me to his wife.
Also note, nothing of what I've said here has to do anything with »darf, muss, kann, ...«
"Ich darf mich vorstellen" parallels the English construction. It means "I must introduce myself," which is "accusative" in both English and German, and therefore uses the "mich" form.
You may be confused because "Ich darf mir ein Essen aussuchen," does not parallel the English construction. The English version would be, "I must seek a dinner," as opposed to "I must seek a dinner for myself."
But Germans are more explicit about recipients, because they might interpret "Ich darf mir ein Essen aussuchen," as "I need to get dinner for someone else."
Hence they add the word "mir" to the sentence, to make it clear that I am getting dinner for myself. And "for myself" is translated using the dative, mir, versus the accusative, mich, when referring only to "myself."
If your native language does not identify pronoun cases by declension but merely distinguishes between subject ('I', 'we') and object pronoun forms ('me', 'us') as English does, a practical way to relate to the bewildering accusative versus dative case distinction is that the former typically denotes the direct object of a sentence and the latter its indirect object. The full rule framework on appropriate case usage is fussier than that simplistic explanation, I'm afraid, but let's stick with the direct / indirect object roles for now.
The sentence ich darf mich vorstellen contains but one object: Yourself, the direct object being introduced. In this example, the fact that you are both subject and object of the case matters little in terms of grammar. If a third person was involved -- doing the introducing, or being the one introduced -- the direct object status remains consistently marked by an accusative pronoun; be it ich darf ihn vorstellen or er darf mich vorstellen. Since you're introducing yourself, the mich pronoun is reflexive.
If you wish to add specificity to your sentence, e.g. by stating to whom you're being introduced, you're adding an indirect object; as in English I may introduce myself to him, which becomes ich darf mich (acc.) ihm (dat.) vorstellen, or with reversed roles, er darf sich (acc., reflexive) mir (dat.) vorstellen.
The other sentence about choosing some food for yourself is also purely about role-play between a direct object (the food chosen), and the indirect object the food is chosen for (namely yourself, as the sentence is also reflexive). Therefore ein Essen as direct object is marked by the accusative case whereas mir (for myself) as indirect object carries dative case markings.
That is, in essence, all that explains the grammatical distinction. Noun and adjective case endings in 'the awful German language' to quote Mark Twain are quite washed out and often rely on an inflected article for clarity, which is basically why this aspect of German grammar is so incredibly difficult to master with fluency - you have my full sympathy! Pronouns however are more distinct and unambiguous; ich-meiner-mir-mich, or er-seiner-ihm-ihn, etc. So it's really a question of mapping form (inflection/declension) over function (direct vs indirect object).
German tends toward heavy use of reflexive verbs. This is nothing particularly German (as in, "so precise...") but is actually common for many Indo-European languages - English, as a matter of fact, is the exception here. For that reason constructions like sich waschen, sich rasieren and sich anziehen for to wash / shave / dress are common. The same applies to sich etwas aussuchen (to pick something for yourself, e.g. birthday dinner). The add-on '...oneself' feels superfluous to English usage, where a direct object to these mundane tasks is only deemed necessary if you're assisting someone else. The absence of a direct object from English marks clearly and unambiguously enough that you are the one being washed, shaved, and dressed... by none other than yourself!
It's worth internalising how English grammar works in contrast to that of other languages. Even though French and Spanish have lost Latin's case inflections, they do still distinguish very clearly between direct and indirect object through pronoun markers, and (often) also use reflexive verbs (se laver / lavarse or s'habiller / vestirse to stick with the above morning routine examples; French, like German, front-loads the reflexive pronoun se whereas Spanish tags it at the end). English does not; reflexivity is implied by the absence of a pronoun.
