When to use sich?

Question

I have this sentence in english Let's buy it (to us)!. In my workbook, translation of this sentence is following : Kaufen wir es uns!

My question is : Can I also use Kaufen wir es sich!? I have already visited this question, but that does not explain if I can freely switch sich and the pronoun in dative case

Answer 1

The reflexive pronouns in German are (accusative/dative where they differ):

• singular: mich/mir, dich/dir, sich
• plural: uns, euch, sich

So "sich" is only used in the third person.

• Ich kaufe mir ein Buch.
• Du kaufst dir ein Buch.
• Er kauft sich ein Buch.
• Wir kaufen uns ein Buch.
• Ihr kauft euch ein Buch.
• Sie kaufen sich ein Buch.

And of course, you can also buy books for someone else: Du kaufst mir ein Buch. (Danke!)

Answer 2

No, this does not work.

It is tricky, because the infinitive form found in the dictionary

sich etwas kaufen

uses sich, but this has to be replaced by the appropriate personal pronoun, matching the earlier occurrence, which in the example case requires the plural uns. Sich is either singular or third person plural and would not match grammatically.

The full list is: ich/mir, du/dir, er-sie-es/sich, wir/uns, ihr/euch, sie/sich.

It is of course possible to buy something for somebody else, as in

Wir kaufen ihm/dir/mir etwas.

but as I understand this is not the scope of your question.