die Tafel
board
A large, usually black or green writing surface in a classroom, written on with chalk.
Der Lehrer schrieb die Hausaufgaben an die Tafel.
The teacher wrote the homework on the board.

board / bar / food bank
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board
A large, usually black or green writing surface in a classroom, written on with chalk.
Der Lehrer schrieb die Hausaufgaben an die Tafel.
The teacher wrote the homework on the board.

The many meanings of 'Tafel' (board / bar / food bank) become clear from context. 'Schule' (school) or 'Kreide' (chalk) points to the writing surface, 'Schokolade' (chocolate) to the sweet, and 'Spenden' (donations) or 'Armut' (poverty) to the aid organization.
In der Schule steht die Lösung an der Tafel.
At school, the solution is on the board.
For the writing surface, the prepositions are important. You write something 'an die Tafel' (on/to the board; accusative, direction), and something is written 'an der Tafel' (on the board; dative, location). A typical verb is 'wischen' (to wipe).
Der Schüler ging nach vorne, um die Aufgabe an die Tafel zu schreiben.
The student went to the front to write the exercise on the board.
In the meaning 'portion', 'Tafel' (bar) is used almost exclusively in the fixed combination 'eine Tafel Schokolade' (a bar of chocolate). It works like a counting unit for chocolate.
Ich habe im Supermarkt zwei Tafeln Schokolade gekauft.
I bought two bars of chocolate at the supermarket.
In Germany, 'Die Tafel' is the name for organizations that distribute food to people in need. You say 'zur Tafel gehen' when someone makes use of help there.
Immer mehr Menschen sind auf die Hilfe der Tafeln angewiesen.
More and more people depend on help from food banks.
As a term for a large, festively set dining table, 'Tafel' (banquet table) is rather elevated or literary. It is used for banquets, weddings, or royal meals, not for a normal kitchen table.
Zur Hochzeit des Prinzen wurde eine prachtvolle Tafel für 200 Gäste gedeckt.
For the prince's wedding, a magnificent banquet table was set for 200 guests.
As a sign, 'Tafel' (plaque / sign) is often used for memorial plaques or information signs on buildings or monuments. Common verbs are 'anbringen' (to put up/attach) or 'enthüllen' (to unveil).
Am Geburtshaus des Dichters wurde eine Gedenktafel angebracht.
A memorial plaque was put up at the poet's birthplace.
table
'der Tisch' and 'die Tafel' can both refer to a place where people eat together. 'Tisch' is the normal everyday piece of furniture. In this meaning, 'Tafel' sounds more ceremonial and usually means a large, beautifully set table for a special meal. The words can be confused because people eat at both. In everyday life, however, people almost always say 'Tisch'. 'Tafel' is more suitable for celebrations, banquets, or elevated language.

Für die Hochzeit war eine lange Tafel mit Blumen und Kerzen gedeckt.
For the wedding, a long banquet table was set with flowers and candles.
Der Mitarbeiter räumt nach dem Gespräch den Tisch im Besprechungsraum ab.
After the meeting, the employee clears the table in the conference room.