zäh
tough
Describes the texture of foods, especially meat, that are firm and hard to chew.
Das Schnitzel war leider so zäh, dass man es kaum schneiden konnte.
Unfortunately, the schnitzel was so tough that you could hardly cut it.

tough / viscous
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tough
Describes the texture of foods, especially meat, that are firm and hard to chew.
Das Schnitzel war leider so zäh, dass man es kaum schneiden konnte.
Unfortunately, the schnitzel was so tough that you could hardly cut it.

You can recognize the meaning of 'zäh' (tough/viscous) from the noun it describes. 'Zähes Fleisch' (tough meat) is hard to chew, 'zähe Verhandlungen' (tough negotiations) are lengthy, 'zäher Honig' (viscous honey) is thick, and a 'zähe Person' (tough person) is persevering.
Nach einem zähen Kampf konnte die Mannschaft das Spiel gewinnen.
After a tough fight, the team was able to win the game.
In a culinary context, 'zäh' (tough) is almost always used negatively for meat or baked goods that are firm and hard to bite or cut.
Das Fleisch war leider trocken und zäh.
Unfortunately, the meat was dry and tough.
This figurative meaning describes processes that move forward slowly and laboriously. Typical collocations are 'zähe Verhandlungen' (tough negotiations), 'ein zäher Prozess' (a tough process), or 'ein zähes Ringen' (a tough struggle).
Die Suche nach einer Lösung war ein zäher Prozess.
The search for a solution was a tough process.
This meaning describes the physical property of thick, sticky substances. It can often be combined with the verb 'fließen' (to flow): 'etwas fließt zäh' (something flows sluggishly).
Der Teig ist sehr zäh und lässt sich nur schwer rühren.
The dough is very viscous and is hard to stir.
When 'zäh' (tough) describes a person, it is usually meant positively. It means that the person is resilient, persevering, and strong-willed.
Sie ist eine zähe Athletin und gibt niemals auf.
She is a tough athlete and never gives up.
hard
'hart' and 'zäh' can both describe something that is not easy. For objects or food, 'hart' means rather firm and not soft, for example bread or wood. 'zäh' is used especially for meat or dough when you have to chew for a long time, or when the mass is difficult to pull or move. For tasks, conversations, or negotiations, 'hart' often means very demanding or strict. 'zäh' emphasizes more that something takes a long time, progresses slowly, and is laborious. Learners may confuse the words because both have to do with resistance. In practice: A 'harter Test' is difficult; a 'zähe Verhandlung' drags on for a long time.

Das Gespräch mit der Versicherung war zäh und dauerte fast zwei Stunden.
The conversation with the insurance company was tough and lasted almost two hours.
Die Prüfung war hart, aber Anna hatte sich gut vorbereitet.
The exam was hard, but Anna had prepared well.