der Kunde
customer
A person or organization that buys goods or uses a service.
Ein zufriedener Kunde kommt gerne wieder.
A satisfied customer is happy to come back.

customer
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customer
A person or organization that buys goods or uses a service.
Ein zufriedener Kunde kommt gerne wieder.
A satisfied customer is happy to come back.

'Der Kunde' is a masculine noun of the N-declension. This means it takes the ending '-n' in all cases except the nominative singular. A common mistake is forgetting this ending.
Der Verkäufer berät den Kunden.
The salesperson advises the customer.
The word 'Kunde' is the basis for many important terms in business. Examples include der Kundenservice (customer service), die Kundennummer (customer number), der Kundenstamm (customer base), or die Kundenbetreuung (customer support).
Bei Problemen rufen Sie bitte unseren Kundenservice an.
If you have problems, please call our customer service.
In everyday business, there are fixed combinations with 'Kunde.' You can win customers, advise customers, or serve customers. A customer can be loyal, new, or potential.
Unser Ziel ist es, neue Kunden zu gewinnen.
Our goal is to win new customers.
guest
'der Kunde' and 'der Gast' can both refer to a person who receives a service. 'Kunde' mainly says that someone buys something or pays for a service. 'Gast' is used when someone is invited or is being served in a hotel, restaurant, or café. In hospitality, the same person can therefore be both a customer and a guest, but 'Gast' sounds more natural and friendly there. The words can be confused because both refer to people receiving something in a shop or restaurant. Remember: when paying and buying are the focus, 'Kunde' fits; when hospitality and invitations are the focus, 'Gast' often fits.

Am Telefon erklärt die Mitarbeiterin dem Kunden den neuen Tarif.
On the phone, the employee explains the new plan to the customer.
Zur Feier begrüßt Lena jeden Gast persönlich an der Tür.
At the celebration, Lena greets every guest personally at the door.