steif
stiff
Hard and inflexible; not easy to move or deform.
Der Karton ist steif genug, um als Unterlage zu dienen.
The cardboard is stiff enough to serve as a backing.

stiff
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stiff
Hard and inflexible; not easy to move or deform.
Der Karton ist steif genug, um als Unterlage zu dienen.
The cardboard is stiff enough to serve as a backing.

You can recognize the meaning of 'steif' (stiff) from the noun it describes. If it refers to a body part, it is about pain or immobility. If it describes a person or atmosphere, it means formal and tense.
Nach der langen Wanderung waren meine Beine ganz steif.
After the long hike, my legs were completely stiff.
In connection with the body, 'steif' (stiff) is often used for specific body parts. Typical combinations are 'ein steifer Nacken' (a stiff neck), 'steife Gelenke' (stiff joints), or 'steife Finger' (stiff fingers).
Vom langen Sitzen am Computer habe ich oft einen steifen Nacken.
From sitting at the computer for a long time, I often get a stiff neck.
When a person or their behavior is described as 'steif' (stiff), it is usually meant negatively or critically. It describes a lack of ease and naturalness in social situations.
Er ist ein sehr steifer Typ und lacht fast nie.
He is a very stiff guy and almost never laughs.
In the context of cooking and baking, 'steif' (stiff) is used almost exclusively in the fixed expression 'etwas steif schlagen' (to beat something until stiff). This usually refers to cream or egg whites.
Für die Mousse au Chocolat musst du die Sahne steif schlagen.
For the chocolate mousse, you have to whip the cream until stiff.
In relation to the weather, 'steif' (stiff) occurs almost only in the fixed combination 'eine steife Brise' (a stiff breeze). This describes a strong but not stormy wind.
Am Meer weht heute eine steife Brise.
There is a stiff breeze blowing by the sea today.
firm / fixed
'fest' and 'steif' can both describe something as not soft or not easy to move. 'fest' is broader and often neutral: a table stands firmly, a lid sits tightly, or a material is solid. 'steif' emphasizes more strongly that something can hardly bend or move. With body parts, 'steif' often means that movement is difficult or painful. With people, 'steif' describes very formal, not relaxed behavior. The words can be confused especially with things that seem hard. As a simple guide: 'fest' means stable or tight, while 'steif' means inflexible or not relaxed.

Nach der langen Fahrt ist mein Nacken ganz steif.
After the long drive, my neck is completely stiff.
Der Deckel sitzt so fest, dass ich das Glas nicht öffnen kann.
The lid is on so tightly that I can't open the jar.