acht
eight
The cardinal number 8, the digit between seven and nine.
Ein Oktopus hat acht Arme.
An octopus has eight arms.

eight
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eight
The cardinal number 8, the digit between seven and nine.
Ein Oktopus hat acht Arme.
An octopus has eight arms.

The word 'acht' (eight) is part of the fixed expressions 'Acht geben' (to watch out) or 'sich in Acht nehmen' (to be careful). Both mean 'to be careful' or 'to watch out' and have nothing to do with the number.
Du solltest dich vor Taschendieben in Acht nehmen.
You should watch out for pickpockets.
The noun 'die Achtung' (caution, respect) is related to 'acht' (eight). When you hear 'Achtung!' (Attention!), it is a warning to watch out.
Achtung, hier ist der Boden nass!
Careful, the floor is wet here!
The expression 'etwas außer Acht lassen' (to leave something out of account) means to ignore something or not take it into consideration. Here too, it is about attention, not the number.
Bei deiner Planung darfst du die Kosten nicht außer Acht lassen.
When planning, you must not ignore the costs.
eighty
'acht' and 'achtzig' are both numbers and sound similar, but they mean very different quantities. 'acht' is the number 8; 'achtzig' is the number 80. Learners may confuse them when listening, especially when people speak quickly. Pay attention to the ending '-zig': it turns a ones number into a tens number. With prices, times, ages, or quantities, the difference is very important.

Herr Hoffmann hat acht Stühle für den Kurs vorbereitet.
Mr. Hoffmann prepared eight chairs for the course.
Die Mitarbeiterin hat achtzig Euro aus der Kasse gezählt.
The employee counted eighty euros from the cash register.