schleichen
to sneak
To move quietly, slowly, and carefully in order not to be heard or seen.
Der Einbrecher schlich durch die dunkle Wohnung.
The burglar sneaked through the dark apartment.

to sneak
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to sneak
To move quietly, slowly, and carefully in order not to be heard or seen.
Der Einbrecher schlich durch die dunkle Wohnung.
The burglar sneaked through the dark apartment.

Pay attention to the auxiliary verb in the perfect tense! For movement from A to B, 'sein' (to be) is used. With the reflexive forms, 'haben' (to have) is used.
Die Katze ist leise ins Zimmer geschlichen.
The cat sneaked quietly into the room.
The verb is used without a reflexive pronoun only in the basic meaning 'to move quietly'. When it means that something happens unnoticed or that someone should disappear, it is reflexive ('sich schleichen').
Er schlich auf Zehenspitzen aus dem Raum.
He sneaked out of the room on tiptoe.
In this figurative meaning, 'sich schleichen' (to creep in) is often used for abstract, negative things. Typical phrases are 'Fehler schleichen sich ein' (errors creep in), 'Zweifel schleichen sich ein' (doubts creep in), or 'eine Gewohnheit schleicht sich ein' (a habit creeps in).
Obwohl er es nicht wollte, schlich sich die alte Gewohnheit wieder ein.
Although he did not want it to, the old habit crept back in.
In this meaning, the verb is very colloquial and often rude. The command 'Schleich dich!' is a very direct and rude way to say 'Go away!'
Als er merkte, dass er unerwünscht war, schlich er sich von der Party.
When he realized he was not wanted, he slipped away from the party.
to disappear
'verschwinden' and 'sich schleichen' can both mean, in a rude command, that someone should leave. 'Verschwinde!' is direct and harsh. 'Schleich dich!' is also colloquial and derogatory, but sounds even more like anger or rejection. Important: 'schleichen' also has a normal meaning, namely to move quietly and carefully. In the meaning 'to leave', it is usually used reflexively or as a fixed command. In neutral situations, neither word is appropriate in this use.

Als der Nachbar laut wurde, sagte er nur: „Schleich dich!“
When the neighbor got loud, he simply said, "Clear off!"
Der ältere Bruder rief wütend: „Verschwinde aus meinem Zimmer!“
The older brother shouted angrily, "Get out of my room!"