schief
crooked / tilted
Not straight or horizontal; tilted or curved.
Das Bild an der Wand hängt ein bisschen schief.
The picture on the wall is hanging a little crooked.

crooked / off-key
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crooked / tilted
Not straight or horizontal; tilted or curved.
Das Bild an der Wand hängt ein bisschen schief.
The picture on the wall is hanging a little crooked.

In the meaning of 'misslingen' (to fail), 'schief' is almost always combined with the verbs 'gehen' (to go) or 'laufen' (to run/go). 'Schiefgehen' (to go wrong) and 'schieflaufen' (to go wrong) are separable verbs in which 'schief' stands at the end of the sentence.
Bei der Organisation des Festes ist einiges schiefgelaufen.
Quite a few things went wrong during the organization of the party.
The expression 'jemanden schief ansehen' (to look askance at someone) is a fixed expression. 'Ansehen' (to look at) is a separable verb here, and the person who is being looked at disapprovingly is in the accusative.
Nachdem er den Fehler gemacht hatte, sahen ihn seine Kollegen schief an.
After he had made the mistake, his colleagues looked askance at him.
The meaning 'unrein' (out of tune) or 'falsch' (wrong) is used exclusively in the context of music, noises, and tones. People say 'schief singen' (to sing off-key) or 'schiefe Töne' (off-key notes).
Die Geige klingt ein wenig schief, sie muss gestimmt werden.
The violin sounds a little off-key; it needs to be tuned.
In its literal meaning, 'schief' is often used to describe the position of objects. Typical collocations are 'ein schiefer Turm' (a leaning tower), 'ein schiefes Lächeln' (a crooked smile), or 'ein Bild hängt schief' (a picture is hanging crooked).
Kannst du das Poster bitte geraderücken? Es hängt total schief.
Can you straighten the poster, please? It's hanging completely crooked.
right now / straight
'gerade' and 'schief' form a clear opposite pair when it comes to shape or position. 'gerade' describes something that is not bent or that runs in a clean line. 'schief' describes something that is not straight or not horizontal, but seems tilted or incorrectly aligned. This is especially typical with pictures, walls, lines, shelves, or paths. Memory tip: if something stands neatly in a line, it is 'gerade'; if it tilts to the side or hangs incorrectly, it is 'schief'.

In der Küche hängt das Regal schief an der Wand.
In the kitchen, the shelf is hanging crookedly on the wall.
Zieh bitte eine gerade Linie unter die Überschrift.
Please draw a straight line under the heading.