mich, dich, sich, uns, euch

Some verbs need a reflexive pronoun that points back to the subject. In the accusative the forms are mich for ich, dich for du, sich for er, sie, es and for polite Sie, uns for wir and euch for ihr. Only the third person has a special form; the others look like the normal object pronouns. So you say Ich wasche mich and Wir treffen uns.

The pronoun must match the subject in person and number. A very common mistake is using sich for every subject, for example Ich habe sich getroffen or Wir haben sich getroffen instead of mich and uns. Note that some verbs take the dative reflexive pronoun (mir, dir) when there is another object, for example Ich kaufe mir ein Buch. That pattern is taught separately.